UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

A new reaction for coordinated dinitrogen MacKay, Bruce Alexander 2004

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-ubc_2004-902242.pdf [ 17.53MB ]
Metadata
JSON: 831-1.0061149.json
JSON-LD: 831-1.0061149-ld.json
RDF/XML (Pretty): 831-1.0061149-rdf.xml
RDF/JSON: 831-1.0061149-rdf.json
Turtle: 831-1.0061149-turtle.txt
N-Triples: 831-1.0061149-rdf-ntriples.txt
Original Record: 831-1.0061149-source.json
Full Text
831-1.0061149-fulltext.txt
Citation
831-1.0061149.ris

Full Text

A NEW REACTION FOR COORDINATED DINITROGEN by B R U C E A L E X A N D E R M A C K A Y B . Sc. (Hons.), University of British Columbia, 1999 A thesis submitted in partial fulfillment of the requirements for the degree of D O C T O R O F P H I L O S O P H Y in T H E F A C U L T Y O F G R A D U A T E S T U D I E S D E P A R T M E N T O F C H E M I S T R Y We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January, 2004 © Bruce Alexander MacKay , 2004 ABSTRACT The chemistry of the tantalum dinitrogen complex [NPN]Ta([A -H) 2 ( |x-T] 1 :Ty 2 -N 2 )Ta[NPN] (where [NPN] = (PhNSiMe 2 CH 2 ) 2 PPh) with main group hydride reagents is explored. The H - E bond of these reagents (E = B , A l , Si) undergoes addition across a Ta-N 31 bond, leading to formation of a new N-E bond and a new tantalum hydride. These complexes undergo spontaneous cleavage of the remaining N-N bond, coupled to reductive elimination of bridging hydride ligands as H 2 . Hydroboration of [NPN|Ta (n -H ) 2 (n-ri 1 :Ti 2 -N 2 )Ta[NPN; | with 9 - B B N in toluene/THF solution cleanly gives |NPN]Ta (H) ( fx -H ) 2 (^ -T i ' :T i 2 -NNBQH 1 4 )Ta [NPN] over 24 h. This complex undergoes reductive elimination of bridging hydrides as H 2 and N-N bond cleavage. A n [NPN] ligand silyl group migrates to the reactive nitrido ligand arising from N - N bond cleavage, giving |NPn -N]Ta(=NPh)(fxN-B(H)C 8 H, 4 )Ta[NPN| . This complex eliminates the borohydride and the phenyl group of the phenylimido ligand as benzene to give |NPfx-N|Ta(=NBQH 1 4 ) ( [x -N)(Ta[NPN| . This complex reacts with an additional equivalent of 9 - B B N via formation of a simple Lewis acid-base adduct rather than by B - H addit ion to give [NP(x-N]Ta(=NBC 8 H 1 4 ) (n -NB(H)QH 1 4 )Ta[NPN]. Homologs for the first three of these complexes are synthesized using dicyclohexylborane in place of 9 - B B N . The benzene elimination event in the dicyclohexylboryl homolog proceeds more cleanly than in the prototypical system. Homologs for the first two of the prototype complexes are prepared using the primary borane thexylborane. Hydroalumination of [NPN|Ta([x-H) 2(fx--r | J:Ti 2-N 2)Ta|NPN] proceeds to give [NPN|Ta(H)(fx-H) 2 (^-NNAl 'Bu 2 )Ta[NPNj quantitatively in seconds. This species rearranges rapidly via p elimination of an isobutyl ligand as isobutylene to give an intermediate complex that is tentatively identified by multinuclear N M R spectroscopy as [NPN]Ta(H)(fx-H) 2 ( jx-NNAl(H) 'Bu)Ta|NPN]. This complex undergoes reductive elimination of H 2 and N - N bond cleavage to give two diastereomers of [ N P N -Xl(H)'Bu]Ta(H)(^-N)(n-N)Ta[NPN|. ii Hydrosi lylat ion of [NPN]Ta<u.-H) 2(u.-V : r i 2 -N 2 )Ta[NPN] with n-butylsi lane proceeds cleanly over 4 to 6 hours to give |NPN; |Ta(H)(u -H) 2 (} i -NNSiH 2 "Bu)Ta |NPN] . This species undergoes facile N - N bond cleavage coupled to H 2 elimination to give nitrido complex [NPN|Ta(H)(u 1 -N)(u . -NSiH 2 n Bu)Ta[NPN; | quantitatively. Homologs of these complexes are available using phenylsilane in place of «-butylsilane. The nitrido complex can be hydrosilylated at the new nitrido ligand with a second equivalent of R-S i H 3 to give ([NPN|Ta) 2 (fx-N-SiH 2 "Bu)(n-N-SiH 2 R) (R = "Bu, Ph, C H 2 C H 2 S i H 3 ) , in which both dinitrogen-derived atoms are s imilar ly functionalized despite their asymmetric activation in the parent dinitrogen complex. In the presence of additional silane, these complexes undergo drastic rearrangement of the T a 2 N 2 core and the ortho-metalation of an | N P N | phosphine's phenyl group to give |NP(Ph)N]Ta("BuSiH 2 -N-Si(H) n Bu-u.-N)Ta|NPN] and related species. ( | ;NPN|Ta) 2 (u-N-SiH 2 "Bu)(u J -N-SiH 2 Ph) can be synthesized independently by the reaction of n-butylsilane with the phenyl homolog of the nitride complex. ( |NPN|Ta) 2 (u , -N-SiH 2 Ph) 2 exhibits | N P N | ligand amide migration to give [NPrJ ; |Ta(^-N-SiH 2 Ph)(^-N-SiH 2 Ph)Ta | ;NPN]. Hydrosilylation of |NPN|Ta([x-H) 2 (u,-N 2 )Ta[NPN] with diphenylsilane proceeds slowly as compared to the N - N bond cleavage process, allowing isolation of nitride complex [ N P N I T a C ^ C f x - ^ C j x - N - S i ^ P h ^ T a l N P N ] . This complex resists second silylations. The bulky primary silane l B u S i H 3 reacts with the parent dinitrogen complex via addition, but rapidly rearranges via amide migration, cyclometalation, phosphine d i ssoc ia t ion , and T a 2 N 2 core rearrangement to give | N P ( P h ) N | T a ( f i - N -S i H 2 l B u)( lB uSi(H)N=)Ta|NPN]. in TABLE OF CONTENTS A B S T R A C T T A B L E O F C O N T E N T S LIST O F T A B L E S LIST O F F I G U R E S G L O S S A R Y O F T E R M S A C K N O W L E D G E M E N T S D E D I C A T I O N Chapter 1. Dinitrogen coordination chemistry and reaction design 1.1. Dinitrogen and fixation 1 1.2. Dinitrogen coordination chemistry 3 1.3. Synthesis of dinitrogen complexes 5 1.4. Reactions of coordinated dinitrogen 10 7.5. Dinitrogen cleavage 12 1.6. Dinitrogen activation in the Fry/.uk laboratory 14 1.7. Structure of 1 and rudimentary discussion of bonding 19 1.8. Nuclear Magnetic Resonance spectroscopy studies 20 1.9. Preliminary reactivity studies 22 1.10. Using molecular orbitals to rationalize reactions 23 / . / / . Scope and prospectus 26 1.12. References 29 Chapter 2. Hydroboration of coordinated dinitrogen 2.1. Introduction 35 2.2. Mechanistic considerations 37 2.3. The case for and against hydroboration of dinitrogen 38 2.4. Hydroboration of 1 using 9 - B B N 42 2.5. Spontaneous transformations of 2.1 45 2.6. 2 y S i N M R spectroscopy as a probe of mechanism 47 iv ii iv vii xi xvi X X xxi 2.7. Isolation of an intermediate 49 2.8. Further addition reactions 53 2.9. A homologous series of complexes with dicyclohexylborane 55 2.10. Homologous complexes with thexylborane 58 2.11. Summary and Conclusions 60 2.12. Experimental Section 63 2.12.1. General Considerations 63 2.12.2. Starting Materials and Reagents 64 2.12.3. Synthesis, Characterization, and Reactivity of Complexes 64 2.13. References 70 Chapter 3. Hydroalumination of coordinated dinitrogen 3.1. Introduction 73 3.2. Hydroalumination of 1 75 3.3. Exploring the transformations of 3.1 using 3 I P N M R spectroscopy 77 3.4. Molecular structure of 3.3 80 3.5. Complexes 3.3 and 3.4 are diastereomers 82 3.6. The fate of the isobutyl group 84 3.7. Nature of intermediate 3.2 ' 8 5 3.8. Interconversion of 3.3 and 3.4 86 3.9. Overall mechanism and kinetics 87 3.10. Reactions of 3.3 with C - C multiple bonds 89 3.11. Summary and conclusions 89 3.12. Experimental Section 91 3.10.1. General Considerations 91 •3.10.2. Reagents 91 3.10.3. Synthesis, Characterization, and Reactivity of Complexes 91 3.13. References 96 v Chapter 4. Hydrosilylation of coordinated dinitrogen 4.1. Introduction 98 4.2. Hydrosilylation of 1 wth /2-butylsilane 100 4.3. Facile N - N bond cleavage 103 4.4. Monitoring hydrosilylation of 1 by 3 ' P N M R spectroscopy 106 4.5. Rearrangements due to excess silane 112 4.6. Hydrosilylation of 1 with phenylsilane 115 4.7. Attempted kinetic studies 123 4.8. Reactions of a bifunctional silane 124 4.9. Effects of steric bulk 128 4.10. Summary and conclusions 132 4.11. Experimental Section 134 4.11.1. General Considerations 134 4.11.2. Reagents 134 4.11.3. Synthesis, Characterization, and Reactivity of Complexes 134 4.12. References 144 Chapter 5 Synopsis and extensions 5.1. Synopsis of reactions 147 5.2. Mechanistic considerations and future kinetic studies 149 5.3. Reactions with cumulene electrophiles 150 5.4. Attempted hydrozirconation of 1 153 5.5. Experimental Section 157 5.5.6. General Considerations 157 5.5.7. Reagents 157 5.5.8. Synthesis and Characterization of Complexes 157 5.6. References 159 Appendix 1 X-ray Crystal Structure Experimental Information Al.l. General Considerations 160 A1.2. References 167 vi LIST OF TABLES Chapter 1 Dinitrogen coordination chemistry and reaction design Table: Title: Page: Table 1.1. N - N bond lengths and stretching frequencies for some 4 simple molecules Chapter 2 Hydroboration of coordinated dinitrogen Table: Title: Table 2.1. Selected bond distances (A), bond angles (°), and dihedral angles (°) for [NPNlTaCHXix-H^Cn -Ti ' iTi^NNB C 8 H 1 4 )Ta[NPN] ,2.1. Table 2.2. Selected bond distances (A), bond angles (°), and dihedral angles (°) for [NPu.-N]Ta(=NBC 8 H l 4 ) (u-N)(Ta[NPN], 2.2. Table 2.3. Selected bond distances (A), bond angles (°), and dihedral angles (°) for [NP^-N]Ta(=NPh)(^NB(H)QH, 4 )Ta[NPN | ,2 .3 . Table 2.4. Selected bond distances (A), bond angles (°), and dihedral angles (°) for | N P u - N ] T a ( = N B C 8 H 1 4 ) ( u NB(H)QH I 4 )Ta |NPN ] , 2 .4 Table 2.5. Selected bond lengths (A), bond angles (°), and dihedral angles (°) for [ N P u - N J T a ^ N P h X u N B C H X Q H , , ) 2 ) T a [ N P N | , 2.6. Page: 43 46 49 54 57 vn Chapter 3 Hydroalumination of coordinated dinitrogen Table: Title: Table 3.1. Comparison of selected bond distances (A), bond angles (°), and dihedral angles (°) for complexes 3.3 and 3.4 Table 3.2. 3 I P N M R spectral data on test reactions between 3.3 and hydrocarbons. Page: 83 95 Chapter 4 Hydrosilylation of coordinated dinitrogen Table: Title: Table 4.1. Selected bond distances (A), bond angles (°), and dihedral angles (°) for [NPN]Ta(H)(M.-H) 2 ( |x -Ti 1 : i i 2 -NNSiH 2 , , Bu) T a | N P N | , 4.1. Table 4.2. Selected bond distances (A), bond angles (°), and dihedral angles (°) for [NPN|Ta(H)( fx-N)([x-NSiH 2"Bu)Ta[NPN;i, 4.2 Table 4.3. Selected bond distances (A), bond angles (°), and dihedral angles (°) for ([NPN]Ta) 2 (^-NSiH 2 "Bu) 2 , 4.3 Table 4.4. Selected bond distances (A), bond angles (°), and dihedral angles (°) for |NP(Ph)N;|Ta( n BuSiH 2 -N-Si(H) n Bu-u J -N)Ta[NPN | ,4.4. Table 4.5. Selected bond distances (A), bond angles (°), and dihedral angles (°) for i;NPNJTa(H)(u-H) 2(u,-r) l:'n 2-N N S i H 2 P h ) T a | N P N | , 4.5. Table 4.6. Selected bond distances (A), bond angles (°), and dihedral angles (°) for [NPN]Ta(jx-NSiH 2Ph)([x-NSiH 2Ph)Ta[NPN;|,4.7. Table 4.7. Selected bond distances (A), bond angles (°), and dihedral angles (°) for |NPN]Ta(^-N-SiH 2 n Bu)(u.-N-S i H 2 C H 2 C H 2 S i H 3 ) T a [ N P N ] , 4.9 Page: 101 105 110 113 117 119 125 viii Table 4.8. Selected bond distances (A), bond angles (°), and dihedral 127 angles (°) for |NP(Ph)N]Ta(N-SiH 2 CH 2 CH 2 Si(H)-fx-N)Ta[NPN],4 .10 Table 4.9. Selected bond distances (A), bond angles (°), and dihedral 131 angles (°) for [NP(Ph)N]Ta( !x-N-SiH 2 'Bu)CBuSi(H)N=)Ta[NPN| ,4 .12. Chapter 5 Synopsis and extensions Title: Table: Table 5.1 Table 5.2. Selected bond distances (A), bond angles (°), and dihedral angles (°) for ( [ N P N | T a ^ S ) 2 ( u ^ C H 2 ) , 5.1 Selected bond distances (A), bond angles (°), and dihedral angles (°) for [N([x-P=N)N]Ta(^-H)2((x-N(ZrCp 2 ) )Ta |NPN| , 5.2. Page: 152 154 Appendix 1 X-ray Crystal Structure Experimental Information Table: Title: Table A - l . Crystallographic Data and Structure Refinement for [NPN)Ta(H))(fx-H) 2 ( } x-ri 1 : 'n 2 -NNBC 8 H 1 4 )(Ta|;NPN] (2.1), [NPjx-N]Ta(=N-BC 8H 1 4)(n-N)(Ta[NPN] (2.2) [NP[x-N]Ta(=NPh)(n-N-B(H)C 8H 1 4)Ta[NPN] (2.3), and [NP(x-N]Ta(=N-BC 8 H 1 4 ) ( f i -N-B(H)C 8 H 1 4 ) (Ta[NPN] (2.4). Table A - 2 . Crystallographic Data and Structure Refinement for [NPM.-N]Ta(=NPh)(n-N-B(H)(C 6H,,) 2)Ta[NPN], (2.6)., an/i-[NPN-Al(H)C 4H9]Ta((x-N)(|x-N)Ta[NPN] (3.3), and 5y/i-[NPN-AI(H)C 4Ha|Ta(n-N)(n-N)Ta[NPN] (3.4). Table A - 3 Crystallographic Data and Structure Refinement for [NPN]Ta(H)(pi-H) 2(ia-Ti 1: 'n 2-NNSiH 2"Bu)Ta[NPN;| (4.1), [NPN]Ta(H)(n-N)(|x-N-SiH 2 nBu)Ta[NPN] (4.2), and Page: 161 162 163 IX ([NPN]Ta) 2(u.-N-SiH 2 nBu) 2 (4.3). Table A-4 Crystallographic Data and Structure Refinement for 164 [NP(Ph)N]Ta( nBuSiN-^i-Si(H) nBu-n-N)Ta[NPN],(4.4), [NPN |Ta(H)(u J-H) 2(u-Ti l:ri 2-NNSiH 2Ph)Ta[NPN],4.5, and [NPN]Ta((x-NSiH 2Ph)(n-NSiHPh)Ta[NPN], (4.7). Table A-5 Crystallographic Data and Structure Refinement for 165 [NPN]Ta (n -NSiH 2 n Bu ) ( (x -NSiH 2 CH 2 CH 2 SiH 3 )Ta[NPN], (4.9) , [NP(Ph)N|Ta(N-SiH 2 CH 2 CH 2 Si(H)-u J -N)Ta[NPN] (4.10) , and [NP(Ph)N]Ta(u.-N-SiH 2 tBu)( tBuSi(H)N=)Ta |NPN] (4.12). Table A-6 Crystallographic Data and Structure Refinement for 166 ([NPN;|Tau-S)2(u.-CH2), 5.1.and [N(u.-P=N)N]Ta(u-H) 2(n-N(ZrCp 2))Ta[NPN], (5.2). x LIST OF FIGURES Chapter 1 Dinitrogen coordination chemistry and reaction design Figure: Title: Page: Figure 1.1 Drawing of the FeMo-cofactor of the nitrogenase enzyme 2 of Acinetobacter vinelandii as determined to 1.16 A resolution. The presence of the central atom (blue) was not suspected until recently. Figure 1.2 Bonding modes for dinitrogen with one and two transition 4 metals. In the dinitrogen unit, N - N activation is implied by bond order Figure 1.3 Metal d orbitals and dinitrogen ligand JT* orbitals of 5 correct orientations for bonding interactions via back-donation in the bimetallic end-on, side-on, and side-on end-on modes of N 2 coordination. Figure 1.4 Summary of Nuclear Magnetic Resonance spectral data 21 for 1 and its isotopomers. Figure 1.5 Representation of an isosurface of the L U M O of a model 24 complex of 1 as calculated by D F T , and a simplified drawing depicting corresponding metal d orbitals. Figure 1.6 Representation of an isosurface of H O M O - 1 of a model 25 complex of 1 as calculated by D F T , and a simplified drawing depicting corresponding metal d orbitals. Figure 1.7 Representation of an isosurface of the H O M O of a model 26 complex of 1 as calculated by D F T , and a simplified drawing depicting corresponding metal d orbitals. xi Chapter 2 - Hydroboration of coordinated dinitrogen Title: Figure: Figure 2.1 Figure 2.2. Figure 2.3. Figure 2.4. Figure 2.5. Figure 2.6. O R T E P drawing (ellipsoids at 50% probability) of [NPN]Ta(H)(M.-H) 2(n-N 2-BC 8H 1 4) T a [ N P N | , 2.1 Si ly l methyl and phenyl ring carbons other than ipso omitted for clarity. Hydrides were modeled using X - H Y D E X . O R T E P drawing of [NPu.-N]Ta(=NBQH 1 4 ) (u-N) ( T a | N P N | , 2.2 (ellipsoids at 50% probability). S i ly l methyl and phenyl ring carbons other than ipso omitted for clarity. 2 9 S i - D E P T N M R spectra of 2.2 (top) and l5N2-2.2 (bottom). The additional coupling in the 5 13.6 ppm doublet indicates one [NPNj silyl group has migrated onto a dinitrogen-derived N atom. O R T E P drawing of |NPu, -N]Ta(=NPhXu-NB(H)C 8 H 1 4 ) T a [ N P N | , 2.3 (ellipsoids at 50% probability). S i ly l methyl and phenyl ring carbons other than ipso omitted for clarity. H I was located in the electron difference map and refined isotropically. O R T E P drawing of [NPu.-N]Ta(=NBC 8 H 1 4 )(u.-NB(H) Q H 1 4 ) T a [ N P N ] , 2.4 (ellipsoids at 50% probability). S i ly l methyl and phenyl ring carbons other than ipso omitted for clarity. H71 was located in the electron difference map and refined isotropically. O R T E P drawing (ellipsoids at 50% probability) of |NPn-N]Ta(=NPh)(^-NB(H)(QH n ) 2 )Ta | ;NPN],2 .6 . Si ly l methyl and phenyl ring carbons other than ipso omitted for clarity. H85 was located in the electron difference map and refined isotropically. Page: 43 45 48 49 54 57 xi i Chapter 3 - Hydroalumination of coordinated dinitrogen Figure: Title: Figure 3.1. 3 I P N M R spectrum of 1 (bottom) prior to addition of D I B A L . Conversion to complex 3.1 (top) occurs rapidly. Figure 3.2. 3 I P N M R spectra (C 6 D f „ 300K) at hourly intervals show the conversion of 3.1 (dark blue) into 3.2 (red), which converts in turn to 3.3 and 3.4 (yellow and turquoise). Figure 3.3. O R T E P drawing of 3.3 (ellipsoids at 50% probability). S i ly l methyl and phenyl ring carbons other than ipso omitted for clarity. H72 and H73 were located in the difference map and refined isotropically. Figure 3.4. O R T E P drawing of the solid-state molecular structure of 3.4 (ellipsoids at 50% probability). Si ly l methyl and phenyl ring carbons other than ipso omitted for clarity. H72 and H73 were modeled using X - H Y D E X . Page: 76 78 80 82 Chapter 4 - Hydrosilylation of coordinated dinitrogen Figure: Title: Figure 4.1. O R T E P drawing (ellipsoids at 50% probability) of the molecular structure of [NPN]Ta(H)(u.-H) 2(u.- ,n. 1:ri 2-N N S i H 2 " B u ) T a [ N P N | , 4.1. Si lyl methyls and phenyl ring carbons other than ipso omitted for clarity. Hydrides were modeled using X - H Y D E X Figure 4.2. O R T E P drawing (ellipsoids at 50% probability) of [NPN]Ta(H)(u-N)(u-NSiH 2 "Bu)Ta| ;NPN|, 4.2. S i ly l methyls and phenyl ring carbons other than ipso omitted for clarity. The hydride was modeled using X - H Y D E X . Figure 4.3. 3 ' P N M R spectra acquired at fifteen-minute intervals over the first three hours of the reaction between 1 and four equivalents of /j-butylsilane. Page: 101 105 107 xiii Figure 4.4. 3 ' P N M R spectra for the entire sixteen-hour experiment. Figure 4.5. ' H / 2 9 S i H S Q C correlational spectrum of 4.3. Figure 4.6. O R T E P drawing (ellipsoids at 50% probability) of the solid-state molecular structure of (| NPN]Ta) 2(jx-N S i H 2 " B u ) 2 , 4.3. S i ly l methyls and phenyl ring carbons other than ipso omitted for clarity. Figure 4.7. O R T E P drawing (ellipsoids at 50% probability) of the molecular structure of [NP(Ph)N]Ta( n BuSiH 2 -NSi(H) n Bu-u.-N)Ta[NPN| 4.4 S i ly l methyls and phenyl ring carbons other than ipso omitted for clarity. Figure 4.8. O R T E P drawing (ellipsoids at 50% probability) of the molecular structure of |NPN]Ta(H)((j,-H) 2(jx-Ti 1:Ti 2-N N S i H 2 P h ) T a | N P N | , 4.5. S i ly l methyls and [NPN] ligand phenyl ring carbons other than ipso omitted for clarity. Hydrides were modeled using X - H Y D E X . Figure 4.9. O R T E P drawing (ellipsoids at 50% probability) of the molecular structure of [NPN]Ta(u-N-SiH 2 Ph)(u . -N-S iH 2 Ph)Ta[NPN| , 4.7. S i ly l methyls and [NPN] ligand phenyl ring carbons other than ipso omitted for clarity. Figure 4.10. O R T E P drawing of the solid-state molecular structure of [NPN]Ta((x-NSiH 2 n Bu)(M--NSiH 2 CH 2 CH 2 SiH 3 )Ta[NPN], 4.9 (ellipsoids at 50% probability). Si ly l methyls and [NPN] ligand phenyl ring carbons other than ipso omitted for clarity. Figure 4.11. O R T E P drawing of the solid-state molecular structure of [NP(Ph)N |Ta(NSiH 2 CH 2 CH 2 Si (H ) - [A -N)Ta |NPN| , 4.10 (ellipsoids at 50% probability). S i ly l methyls and [NPN] ligand phenyl ring carbons other than ipso omitted for clarity. 108 109 110 112 116 119 125 127 xiv Figure 4.12. O R T E P drawing of the solid-state molecular structure of 4.12, [NPCP^NJTaCM-N-SiHj 'BuX'BuSiCHJN^TatNPN] (ellipsoids at 50% probability). Si ly l methyls and [NPNJ ligand phenyl ring carbons other than ipso omitted for clarity. 129 Chapter 5 - Summation and extensions Figure: Title: Page: Figure 5.1. O R T E P drawing of the solid-state molecular structure of 151 ([NPN]Ta[i-S) 2([x-CH 2), 5.1 (ellipsoids at 50% probability). Si lyl methyl and phenyl ring carbons other than ipso omitted for clarity. Figure 5.2. O R T E P drawing of the solid-state molecular structure of 154 [N(n-P=N)N]Ta(M.-H) 2(jx-N(ZrCp 2))Ta[NPN],5.2 (ellipsoids at 50% probability). S i ly l methyl and phenyl ring carbons other than ipso omitted. xv GLOSSARY OF TERMS The following abbreviations, most of which are commonly found in the chemical literature, are used in this thesis. Term: Definition: [NPN] ( P h N S i M e 2 C H 2 ) 2 P P h 2 ligand [P 2 N 2 ] P h P ( C H 2 S i M e 2 N S i M e 2 C H 2 ) 2 P P h 2 - ligand [PNP] ( R P C H 2 S i M e 2 ) 2 N ligand ° degrees in measure of angles °C degrees Celsius " B boron-11 1 3 C carbon-13 l 5 N nitrogen-15 / " , A r 2 -W Isotopomer of complex W prepared from 1 selectively 99% enriched at the N 2 l igand ' H proton ' H / ' H C O S Y proton-proton correlational N M R experiment 2 9 S i silicon-29 9 - B B N 9-borabicyclo|3.3.1 |nonane A Angstrom, 10 1 0 m A n absorbance at wavelength n A r general aryl group B B I broad-band inverse b, br broad C, , C s , C 2 h Schoenflies symmetry designations cm centimeters cm"1 reciprocal centimetres Cp cyclopentadienyl ligand C p * pentamethylcyclopentadienyl ligand C p " disubstituted cyclopentadienyl ligand Cy cyclohexyl substituent xvi d" d electron configuration for a transition metal d doublet D2-Yf Isotopomer of complex W prepared from 1 selectively enriched in the bridging positions dd doublet of doublets ddd doublet of doublets of doublets D E F T Distortionless Emission by Polarization Transfer D F T density functional theory dppe £/s(diphenylphosphino)ethane E "entgegen" or trans EI /MS electrospray ionization mass spectrometry equiv or equivs equivalent or equivalents equiv or equivs equivalent or equivalents F W H M full width at half measure, line width a e> grams G C / M S gas chromatography mass spectrometry G P C gas phase chromatography HIPT hexa-isopropylterphenyl substituent H M Q C Heteronuclear Multiple Quantum Coherence H O M O Highest Occupied Molecular Orbital H O M O - 1 molecular orbital of next lowest energy to the H O M O H S Q C Heteronuclear Single Quantum Coherence H z Hertz, unit of frequency P r isopropyl substituent K degrees Kelvin rate constant, observed rate constant I I I ' general ligands or substituents L U M O Lowest Occupied Molecular Orbital M general metal m multiplet M molar M = N nitrido ligand bound to a general metal M X V I I m/Z M e mg M g A D P M g A T P M H z mmol M O s n-" B u o r B u " E l ' H } "I J A B nm o-O R T E P P Ph Pi ppm R S s t T, l B u T H F T M S vt X X - H Y D E X mass to charge ratio methyl substituent milligrams adenosine diphosphate, magnesium salt adenosine tripohsphate, magnesium salt Megahertz, one million Hertz millimoles molecular orbitals normal (as in «-butyl) normal butyl substituent in N M R spectroscopy, observe nucleus "E while decoupling proton in N M R spectroscopy, coupling constant between nuclei A and B over n bonds (n omitted i f not known) nanometers ortho position Oak Ridge Thermal Ellipsoid Program general monodentate phosphine phenyl substituent inorganic phosphate, P0 4 3 " parts per million general alkyl substituent solvent or donor (or sulfur, depending on context) singlet triplet (1:2:1 unless otherwise specified) longitudinal relaxation in N M R tert-butyl substituent tetrahydrofurann trimethylsilyl substituent virtual triplet general halide hydride location program (see Chapter 2 Ref. 55 page 72) xviii Z "zusammen" or cis 6 delta, chemical shift £ extinction coefficient in M ' 1 cm' 1 TJ" hapticity of order n X m a x wavelength of maximum absorbance fx-X bridging X ligand v N N stretching frequency of N - N bond xix ACKNOWLDEGEMENTS The most direct assistance I received in this research came from my supervisor, Professor Michael Fryzuk. He allowed me a great deal of freedom in my chemical investigations, which maintained my interest and my enthusiasm for this work. He was also unafraid to criticize my writing when my freedom-taking began to work against me! Secondly, I must also thank Dr. Samuel Johnson, who was my mentor when I began my studies. The research project described in this thesis would not exist without his original discovery of the tantalum dinitrogen complex of interest, and he is still teaching me about professionalism in science. M y labmates, present and past, have also been fine colleagues and exceptional people to know. The U B C Department of Chemistry has been very kind to me, and the Departmental staff is one of its biggest strengths. I thank Drs. Nick Burlinson ( N M R ) and Brian Patrick (X-ray crystallography) for helping me listen to what the molecules were telling me. Peter Borda and Minaz Lakha (elemental analysis), Marshall Lapawa (mass spectrometry), Liane Darge and Marietta Austria ( N M R ) , Steve Rak and Brian Ditchburn (technical glassblowing) also supported my efforts. The staff of the Mechanical Shop, Electronics Services, Stores, and the office personnel all made it easier for me to conduct my research and to enjoy the past four years at U B C . I was supported less directly, but certainly no less importantly, by my parents, my parents-in-law, my brother Doug and my new sister-in-law Amanda, Sheridan the Artiste and Patrick the Elbow (viva la Tripod!), and most of all by the untiring love and reckless indulgence of my wife, Jodi. I appreciate the financial support of N S E R C (PGS-A and P G S - B ) , the University (Graduate Teaching Award), and the Department (Laird and McDowel l Fellowships). xx DEDICATION "The whole of science is nothing more than a refinement of.everyday thinking" Albert Einstein Physics and Reality, 1936 For Jodi, every day. xxi Dinitrogen coordination chemistry and reaction design 1.1. Dinitrogen and fixation Dinitrogen, N 2 , makes up 78% of the Earth's atmosphere. It is one of the most abundant, available, and inexpensive small molecule feedstocks in the world, and it is an essential element to all life. 1 Yet N 2 is so unreactive that it is employed as an inert gas in chemical synthesis and in the food industry. When nitrogen is converted from the zero oxidation state of the elemental form into bioavailable or chemically useful compounds such as nitrate or ammonium ions, or ammonia, it is said to be ' 'fixed". There are two main nitrogen fixation pathways that connect the biosphere and the atmospheric inorganic reservoir of nitrogen atoms. The natural method is the fixation of N 2 to ammonia by a few species of bacteria including Azotobacter vinelandii and Clostridium pasteuraniumf* This process employs a metalloenzyme with an iron-sulfur-molybdenum cofactor, or FeMoco, at its active site. 4 , 5 Alternative nitrogenase enzymes using vanadium or iron in place of molybdenum are known but are less common. 6 In the nitrogenase reaction, dinitrogen is reduced and protonated at ambient pressure and temperature as shown in Equation 1.1. N=N + 8 H + + 8 e" + 16 M q A T P — (1.1) 2 N H 3 + H 2 + 16 M g A D P + 16 Pj Page 1 References begin on Page 29 Chapter I - Dinitrogen coordination chemistry and reaction design The mechanism of this reaction is still unknown, 7 and Equation 1.1 merely shows the limiting stoichiometry. Bacterial nitrogen fixation is currently the only example of dinitrogen acting as a substrate in the metabolism of any organism. Figure 1.1 shows a drawing of the FeMo-cofactor as indicated by a recent crystal structure of the enzyme. 8 This study located a bridging atom, likely nitrogen, at the center of the FeMo-cofactor. This central site was previously thought to be unoccupied and therefore many considered it the likely site of N 2 binding and reduction. 9 Evidence that the bridging atom is a nitrido ligand that does not exchange during catalysis has renewed interest in the enzymatic mechanism. 1 0 Figure 1.1. Drawing of the FeMo-cofactor of the nitrogenase enzyme of Acinetobacter vinelandii as determined to 1.16 A resolution. The presence of the central atom (blue) was not suspected until recently.8 The other major fixation pathway is via industrial processes that were developed to ensure access to nitrogen compounds, which are important in industry and as fertilizers in agriculture. The most successful example of industrial nitrogen fixation is the Haber-Bosch process," 1 4 which combines N 2 and H 2 at high temperatures and pressures over metal catalysts to produce ammonia as shown in Equation 1.2. This process uses H 2 as both reducing agent and substrate. Haber and Bosch each received a Nobel Prize for discovery and refinement of this technique.' 3"' Haber plants are estimated to use 1% of the world's annual power production. 1 7 They generated 52 mill ion metric tonnes of N H , in the U S , Canada and China in 2002. 1 X Page 2 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design F e or R u c a t a l y s t N E N ( g ) + 3 H 2 ( g ) — - — 2 N H 3 ( g ) n 2 , 1 0 0 to 3 0 0 a t m 0 -2 ) 4 0 0 to 5 5 0 °C AH°= - 4 6 . 2 k J m o l " 1 , AS°= - 9 9 J m o l " 1 K" 1 1.2. Dinitrogen coordination chemistry The nitrogen reduction pathways described above rely on transition metal catalysts. For this reason, comparisons to nitrogenase and the Haber process are common in dinitrogen coordination chemistry although few useful synthetic models of either process ex is t . 1 4 1 7 , 1 9 , 2 0 Dinitrogen is not a good ligand for transition metals - it is non-polar, is a poor a donor and jr acceptor, and has a large H O M O - L U M O gap.2 1"2 3 Because of these attributes, it is difficult for N 2 to bind to transition metals and difficult for metals to reduce or oxidize complexed N 2 . By comparison, the isoelectronic C O molecule binds metals easily. The ubiquity of carbonyl complexes has allowed development of a rich coordination chemistry with many productive reactions. 2 4 2 3 Dinitrogen chemistry had a slower start - the first dinitrogen complex, | (H 3 N) 5 Ru(N 2 ) J 2 + , was reported in 1965, and its dinitrogen ligand was accidentally derived from hydrazine rather than N 2 . 2 6 2 7 Today, well characterized dinitrogen complexes of every transition metal in groups 4 through 9 have been identified. Dinitrogen complexes of many lanthanides and actinides have also been prepared.28"31 Dinitrogen is typically "activated" when bound to a transition metal. The degree of activation of coordinated N 2 is typically measured by N - N internuclear distance (determined crystallographically) or the v N N stretching frequency in the infrared or Raman spectra. These measures can be loosely correlated to changes in bond order, although some caution is required in these assessments. Table 1.1 summarizes some data on N - N triple, double, and single bonds in representative simple compounds. Activated N 2 units are often referred to as diazenido (N22~) or hydrazido ( N 2 4 ) based on their relationships to these compounds. Page 3 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design Table 1.1. N - N bond lengths and stretching frequencies for some simple molecules Compound: N - N bond length (A): v N N (cm"1): N 3 2 3 3 1N2(g) 1.0975 2331 PhN=NPh 3 3 " 3 6 1.255 1442 H 2 N - N H 2 3 3 ' 3 7 ' 3 * 1.449 1111 Activation and subsequent reactivity of the N 2 unit are both affected by the metal-dinitrogen bonding mode, which varies with the metal, the metal's oxidation state, and the nature of ancillary ligands in the complex. 3 9" 4 1 Historically, the monometallic end-on type of bonding has been prevalent, especially in late metal complexes. These generally display weak dinitrogen activation. Strong activation is more characteristic of higher valent early metal complexes, which show a variety of bonding modes. The known bonding motifs for dinitrogen with either one or two transition metals are shown in Figure 1.2. There is currently only one example of side-on bonding to one transition metal, and this is a metastable form made by photoisomerism of an end-on complex. 4 2 Weak Activation Strong Activation End-On Monometallic M- -N=N M=N: (unknown) =N End-on Bimetallic M N=N- -M M=N—N=M Side-on Bimetallic I < W > N M!T || M N M N .N M* M N > M Side-on End-on Bimetallic M. .M N N' M, I N' :N M (unknown) Figure 1.2. Bonding modes for dinitrogen with one and two transition metals. In the dinitrogen unit, N - N activation is implied by bond order. Pai>e 4 References begin on Page 29 Chapter I - Dinitrogen coordination chemistry and reaction design Because N 2 is a poor a donor, back-donation of electron density from metal d orbitals into it* orbitals on the dinitrogen ligand is important to stabilizing dinitrogen complexes. In a transition metal dinitrogen complex, back-donation often leads to significant elongation of the N - N bond, giving rise to reduction or activation of N 2 . 4 3 , 4 4 Simple bonding interactions clarifying the metal-dinitrogen orbital overlaps for the bimetallic modes are illustrated in Figure 1.3. Complexes with strongly activated N 2 ligands have more electron density in the molecular orbitals resulting from these interactions.4 3"4 8 End-on mode: l_3 V / Side-on mode: c r M Side-on end-on mode: n>-- • .-ft P 1 Figure 1.3. Metal d orbitals and dinitrogen ligand J I * orbitals of correct orientations for bonding interactions via back-donation in the bimetallic end-on, side-on, and side-on end-on modes of N 2 coordination. 1.3. Synthesis of dinitrogen complexes Because its discovery was serendipitous, the synthesis of the first dinitrogen complex did not lead directly to widespread synthesis of other complexes, although it did stimulate this research. Successful synthetic methodologies were developed empirically. 4 9 Weakly activated dinitrogen complexes are sometimes available through ligand Page 5 • References begin on Page 29 Chapter I - Dinitrogen coordination chemistry and reaction design substitution reactions. Since dinitrogen is not a good ligand, typical leaving ligands are weakly bound H,,"*0°2 a cyclometalated ancillary ligand," 1 3 M or an agostic interaction with an ancillary l igand.^^' This reaction clearly depends on a particular metal complex's affinity for N 2 versus the leaving ligand, because there are also cases of N 2 displacement by H 2 5 7 " 5 9 and rare reports of substitution of acetonitrile by N 2 . : ' 0 ' 6 0 Zero-valent complexes of group 6 and 8 metals with four phosphine donors were an important early class of complex. These weakly activated complexes were prepared by reducing metal salts in the presence of phosphines and N 2 as shown in Equation I.3. 6 '" 6 4 These [(P) 4 M(N 2 ) 2 ] complexes and their derivatives are still under study. ( ° ' 6 6 M0CI4 Na/Hg, 2 N 2 2 R 2 K + P R 2 - 4 NaCl r p ^ M o c : p ~ 3 ( i . 3 ) \ / III Page 6 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design One current and widely employed synthetic technique is the reduction of a metal halide complex that already includes the ancillary ligand to give a low-valent intermediate that binds and activates N 2 . The linear bridging iron complex shown at left in Equation 1.4 is an example. This complex was prepared by reduction of [(nacnac)FeClJ in the presence of N 2 . It accepts additional electrons into an orbital that is antibonding with respect to the dinitrogen fragment, and the N - N bond length increases from 1.182(5) A to 1.239(4) A . 6 7 Many early metal N 2 complexes are prepared by this method, and they generally show higher levels of dinitrogen activation than those of late metals. Often, they are bimetallic species with bridging nitrogen ligands as shown in Figure 1.2. A notable example of a highly activated linear bimetallic complex is shown in Equation I .5. 6 8 The product in Equation 1.5 reacts with acetone to give M e 2 C = N - N = C M e 2 and with H + to give good yields of hydrazine, although this type of complex is not generally reactive at the dinitrogen moiety. P M e 3 r> 4 Na/Hg ( M esf f/ <Bu N 2 B u ' ^ 2 TaCI 3 (=CH t Bu)(PMe 3 ) 2 ^ T a = N — N = T a ' (1.5) C 0 Me 3 i p X C 1 P M e 3 Page 7 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design Equation 1.6 shows formation of a heterobimetallic linear N 2 complex. 6 9 The product of Equation 1.6 has been used in preparation of a family of heterobimetallic dinitrogen complexes by salt metathesis with metal halides. 7 0 A neutral monomer, [ ( N 3 N ) M o - N 2 l , has also been isolated. 7 1 Similar studies have been performed using magnesium salts of |.(P-P) 2M(N 2)] ( M = M o , W) . 72 .TMS N' N—Mo—CI • ' N - T M S TMS Mg, N 2 IN IVK T H F .TMS •|\T N—Mo—N=N-T M S TMS -Mg(THF)2 (1.6) Some early metal complexes eliminate hydrocarbons to give coordinatively unsaturated, reduced intermediates that activate dinitrogen. This method has been used to synthesize a known complex with three dinitrogen ligands and two zirconium centers as shown in Equation 1.7.13 The previous route was via reduction of [Cp* 2 ZrCl 2 ] under N ^ 7 (1.7) A related study used 1,3-Zu.s-(TMS)-substituted Cp ligands (Cp") to generate a dinitrogen complex by reduction of alkyl substituents. Alkylat ion of C p " 2 Z r C l 2 with two equivalents of /-butyllithium led to elimination of isobutene and isobutylene, producing a side-on bridging dinitrogen complex in low yield, as shown in Equation 1.8.74 This complex features a very long N - N bond (1.47(3) A) , which suggests an N 2 4 " or hydrazido ligand. However, it is paramagnetic. This indicates that the dinitrogen moiety in this complex may be a rare example of N 2 3 " despite the molecular symmetry. Page 8 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design TMS TMS 2 tBuLi, N 2 2 Cp"ZrCI2 - HCMe 3 -H 2C=CMe2 Hydride ligands can be eliminated as H 2 in dinitrogen complex formation, although this is rare for early metals. The Ti-Me bond of £ is-tetramethyl-cyclopentadienyl(methyl)titanium(III), | (C 5 Me 4 H) 2 TiMe| , reacts with H 2 via hydrogenolysis to give a hydride. Two of these complexes react with N 2 via loss of H 2 to form a linear bridging dinitrogen complex.73 The N, moiety in this complex is weakly bound and can be removed under vacuum to give titanocene. Two equivalents of a substituted <msa-bridged monomeric zirconocene dihydride activate N 2 strongly and in the side-on fashion as shown in Equation 1.9. In contrast to the one shown in Equation 1.8, this complex is diamagnetic.76 B uL Bu' Bu' Dimeric complexes with metal-metal bonds can also activate N 2 . A dimeric calix(4)arene anion of niobium with a Nb-Nb triple bond binds and activates N 2 , giving a linear bridging anionic complex.77 As shown in Equation 1.10, this complex can be reduced to cleave the N-N bond. TMS Me2Si. S i v P ^ l ^ i ™ S SiMe 2 (!•*> Page 9 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design (1.10) 1.4. Reactions of coordinated dinitrogen Many N 2 complexes react with other reagents via loss of N 2 . : , 8 ' : ' 9 , 7 8 However, reactivity at the coordinated N 2 unit is possible. Because of the importance of back-donation in N 2 binding, simple charge considerations predict that coordinated dinitrogen should react with electrophiles. Studies of protonation of [(P) 4 M(N 2 ) 2 | - type complexes introduced in Section 1.3 led to identification of a number of proposed intermediates in biological nitrogen fixation. Joseph Chatt originally proposed a hypothetical cycle based on these complexes as a model for the nitrogenase reaction. The Chatt cycle is shown in Scheme 1.1.79 8 0 Page 10 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design N III N nitrido Scheme 1.1 The names shown for the series of reduced dinitrogen fragments in this cycle are widely employed in dinitrogen coordination chemistry to describe activated and functionalized dinitrogen ligands. The Chatt cycle inspired protonation of N 2 complexes to give ammonia and/or hydrazine, and this is a common test of N 2 activation. < > 5 , 8 1 Even though it is not very effective, the homogeneous nitrogen fixation catalyst mentioned in the last section is a notable result in that it is the first functional Chatt cycle catalyst. Protonation has become a mature reaction pathway, but it is still a benchmark reaction in dinitrogen coordination chemistry, 2 8 , 3 9 8 2 and novel sources of FT such as metal hydrosulfides 8 3 and hydrides 8 4 available from H 2 activation are still being explored. Other electrophiles are now being used in dinitrogen functionalization. Some dinitrogen Page II References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design complexes react with acid chlorides to form N - C bonds8"1 and silyl halides to form N - S i bonds. 8 6 T M S C l , N 2 , and Na , combined with a catalytic amount of a molybdenum dinitrogen complex, can produce si lylamines. 8 7 Y ie lds , selectivity, and turnover frequency are poor due to cross-reactions between the electrophile and the reducing agent. A poorly understood N 2 - T i C l 4 - L i - T M S C l mixture fixes dinitrogen, and has resulted in the formation of N-heterocycles in stoichiometric and catalytic fashion from reactions with specific organic electrophiles such as diketones. 8 8" 9 0 This N atom source has been used in palladium coupling reactions to synthesize organic amides and natural products.9 1"9 3 Neutral [ (P -P ) 2 M(N 2 ) 2 | complexes have been used to prepare nitrogen heterocycles in reactions with similar organic electrophiles. 9 4 9 5 Thus, N 2 activated by transition metals can be incorporated into other substrates, providing the electrophilic reagents are carefully matched to the N 2 complex and the reducing agent. 1.5. Dinitrogen cleavage It has been observed in biological nitrogen fixation that " i f dinitrogen isn't broken, it can't be f ixed ." 9 6 There are now many well-characterized examples of dinitrogen cleavage to give nitride ligands in transition metal chemistry. Three-coordinate complexes of Mo(III) spontaneously bind and reduce dinitrogen. T w o equivalents of | ( R ( A r ) N ) 3 M o | bind N 2 linearly before N - N bond cleavage. 9 7 The product is a Mo(VI) nitride complex, as shown in Scheme 1.2. This study confirms that six reducing equivalents are required for overall cleavage of the N - N triple bond. Symmetry considerations and D F T calculations indicate that bond cleavage proceeds through a zig-zag transition state. 9 8 , 9 9 Page 12 References begin on Page 29 R \ R /R Chapter 1 - Dinitrogen coordination chemistry and reaction design N - M o _ / Ar Ar N^Ar Ar Ar \ Ar / R N " - M O \ ' I R R N N R R N Ar R / Ar N f N Ar \ Ar / Ar Ar Ar \ N R \ N / Ar V Ft N * - M o / il R N N ,R ll / R Mo N P Xi* N Ar \ Ar Ar / R R N ' N III . M o / R Ar ("•"INI \ Ar \ Ar Scheme 1.2 There is considerable interest in nitrogen atom transfer reactions using metal nitrido ligands, but none of these systems use dinitrogen as a nitrogen source. 1 0 0"' 0 2 Despite the fact that the molybdenum nitrido complex in Scheme 1.2 has been known for several years, only recently was its nitrido ligand incorporated into an organic molecule. 1 0 3 Transition metal nitrido complexes are usually unreactive due to the strength of the M s N bond and the stability they impart to the metal complex. The square N b 2 N 2 motif shown in Equation 1.10 is characteristic of group 5 nitride complexes. 3 9 1 0 4 Another example is the reduction of [ (N 2 N)VC1 | , which gives a vanadium(II) intermediate that cleaves N 2 and forms a dimeric nitride as shown in Equation 1.11. Additional reduction affects the vanadium-(N 2N) bonding rather than the bridging nitrido ligands 105 n V i N 2 ' 2 K c s . / V . \ / *• N^ / N (1.11) ™ S - < J ^ T M S T M S - M Q , ? . - T M S j T M S Page 13 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design A variant of the | ( N 3 N ) M o - N 2 | complex shown in Equation 1.6, altered with sterically demanding H1PT (hexa-isopropylterphenyl) substituents on its ( N 3 N ) ligand amido N atoms, was shown to stabilize many Chatt cycle intermediates, including those resulting from N - N bond cleavage. 1 0 6 Experimentation with kinetically compatible reducing agents and proton sources has led to modest yields of ammonia from dinitrogen (four turnovers are reported before the catalyst loses function). 1 0 7 This demonstrates the catalytic cleavage of N 2 via stepwise reduction and protonation, including cleavage of the metal-nitride bond. This scheme also suggests that a single molybdenum center might be able to mediate the nitrogenase reaction. 1.6. Dinitrogen activation in the Fryzuk laboratory The use of mixed-donor ligands incorporating "hard" amido and "soft" phosphine donors' 0 8 is a governing theme in the dinitrogen chemistry explored in this laboratory. These hybrid ligands can be applied across the transition series' 0 9 and to the lanthanides." 0 The pro to typica l an ionic [ P N P ] l igand (where [ P N P ] = ( (R 2 PCH 2 SiMe 2 ) 2 N)~) is available in good yield as an isolable lithiated precursor that can be introduced into the coordination sphere of a metal by salt metathesis with a metal halide as shown in Scheme 1.3, which details the synthesis of | P N P ) Z r C l 3 . ' " Page 14 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design Scheme 1.3 The dinitrogen complex in Scheme 1.3, ( [PNP |ZrCl ) 2 (u^ 2 : - r | 2 -N 2 ) features the side-on bridging mode of bonding, and the N - N bond is elongated beyond that of hydrazine." 2 This complex can be formulated as having two zirconium(IV) centers and a hydrazido, or K , 4 " , ligand. The observation that it does not react cleanly with electrophiles in trial reactions has been attributed to phosphine dissociation, which was observed in a related complex." 3 A strategy aimed at l imit ing this phosphine dissociation led to design and synthesis of the [P 2 N 2 J mac rocyc l i c l igand precursor (where | P 2 N 2 | = P h P ( C H 2 S i M e 2 N S i M e 2 C H 2 ) 2 P P h ) 2 ) as shown in Scheme 1.4."4 Because of the stereochemical possibilities at phosphorus, two isomers are possible. The syn isomer can be prepared exclusively by judicious choice of solvent and temperature. Although macrocycle synthesis usually requires dilute conditions,""' 1 1 6 the synthesis of | P 2 N 2 | benefits from li thium templating, and thus the lithiated ligand precursor is readily available in good yield and purity without high dilution. Page 15 References begin on Page 29 Chapter I - Dinitrogen coordination chemistry and reaction design Me ,Si M e 2 Si . n o CI H ci 2 PhPHLi M e 2 ,Si M e 2 Si . Et 2 0 2 LiCI 3 M r P h - P H (CICH 2 SiMe 2 ) 2 NH P - P h I H 4 BuLi, THF M e 2 ,Si M e 2 Si Ph* — — v M e 2 •Si M e 2 S i . Si M e 2 *Si M e 2 •Ph S = THF P h " •Ph 'S i M e 2 Si M e 2 anti- L i 2 (THF) 2 [P 2 N 2 ] syn-Li 2(THF)[P 2N 2] Scheme 1.4 The complex [ P 2 N 2 ] Z r C l 2 is prepared by salt metathesis, and reduced with K C 8 under N 2 to give the side-on dinitrogen complex ( | P 2 N 2 | Z r ) 2 ( [A - r p r f - N 2 ) . The dinitrogen ligand in this complex is less activated than in the related [PNP] complex ( N - N = 1.43(1) A) . Despite lower N 2 activation, this complex reacts cleanly with the electrophile n-butylsilane as shown in Equation 1.12.1 1 7 Hydrogen gas reacts in the same fashion as n-butylsilane, giving an N - N - H moiety and a bridging hydrido ligand. 4 8 M e 2 M e 2 M e 2 M e ? M e 2 M e 2 , S i . ^ S i M e 2 M e 2 S i . Ph / \ Ph Ph Z r / \ N N \/ N V " S l S i ^ S i M e 2 M e 2 M e 2 M e 2 "BuS iH , Ph \ / Ph B u H 2 S i ^ i r / Ph V / \ Ph (1.12) \ / Ph Z r , V S i s^si M e 2 M e 2 M e 2 M e 2 Page 16 References begin on Page 29 Chapter I - Dinitrogen coordination chemistry and reaction design It was thought that [P 2N 2 | ligand complexes of a group 5 metal would give d 3 systems isoelectronic with the V and Mo complexes previously described, in which N-N bond cleavage was reported.9 7 I t o |P 2 N 2 |NbCl was synthesized, but reduction of this species under N 2 gives a linear paramagnetic bridging diazenido complex, (|P2N2|Nb)2(u.-r|':r|'-N2), as shown in Equation 1.13, rather than a nitrido complex."8 (1.13) Me 2 Me2 M e 2 Me2 Alkylation of [P 2N,|NbCl with methylmagnesium chloride gives paramagnetic d 2 [P,N2]NbMe. This complex binds N 2 reversibly, giving a linear bimetallic dinitrogen complex."9 The fact that structurally similar dinitrogen complexes, rather than nitride complexes, are obtained from both the d 2 and d 3 systems indicates that [P 2N 2|Nb derivatives are not likely to cleave dinitrogen. Niobium analogs of |(R(Ar)N)3Mo], which bind and cleave N 2 , do not display any dinitrogen chemistry, suggesting the problem may be intrinsic to niobium. 1 2 0 1 2 1 |P 2N 2JV supports a dinitrogen complex isostructural to the one shown in Equation 1.13.122 The central goal of Samuel Johnson's doctoral research in the Fryzuk laboratory was to develop dinitrogen activation on tantalum, hopefully without forming unreactive linear bimetallic complexes. A more lofty secondary goal was to develop a dinitrogen activation catalyst.123 iP 2N 2]TaCl 3 proved impossible to synthesize, but the macrocycle has been incorporated into the tantalum complex |P 2 N 2 |TaMe 3 , 1 2 4 which is a platform for organometallic chemistry.Hydrogenation gives the dark purple tantalum(IV) dimeric Page 17 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design hydride complex ( |P 2 N 2 |Ta) 2 (fx-H) 4 , featuring a metal-metal bond. This complex does not react with ethylene or carbon monoxide, and is remarkably stable to oxygen. 1 2 6 . It was hypothesized that this lack of reactivity was due to coordinative saturation - tantalum is eight-coordinate in this complex. To treat this problem, the dianionic [NPN] ligand (where [NPN] = ((PhNSiMe 2 CH 2 ) 2 PPh 2 ~) was designed and synthesized in an extension of the ligand design program described in this section. The ligand synthesis, shown in Scheme 1.5, allows for variation of R groups at amide or phosphine donors independently. The yellow product of the metathesis reaction with T a M e 3 C l 2 is available in good yield, although it was unstable towards thermolysis and photolysis. 2 M e 2 p t n o G 2 P h S i E t 2 ° . S i / K n y V , + 2 L i N H P h • 2 S  V N X [ ( - 2 U C I ) [ H CI CI P H 2 P h 4 BuL i T H F Ph P h \ N . . , * & / P h . / N ^ t . - > » M e T a M e 3 C I 2 ' J t-9\\r.\\ P h — P - ^ L i L r b SiT / ^ M e ( - 2 U C I ) Ph~f u^ P h Scheme 1.5 S x M e , S = T H F Upon hydrogenation of [NPN]TaMe 3 , dark purple ([NPN]Ta) 2 (u,-H) 4 , a diamagnetic Ta(IV)-Ta(IV) complex with a metal-metal bond, is available in quantitative yield. This material gradually turns dark brown under N 2 in a glove box. The brown product is a dinitrogen complex with a structure as shown in Equation 1.14. /'age 18 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design Ph Ph This dinitrogen complex, ( |NPN|Ta) 2 (u.-H) 2 (u.-ri 1 :Tf-N 2 ) , henceforth known as complex 1, is the substrate for the research in this thesis. This side-on end-on mode of bonding in this complex is unprecedented in the literature. A previous trimetallic complex of titanium with a superficially similar shape was not available as a discrete molecule, and its characterization and charge remain uncertain. 1 2 7 1.7. Structure ofl and rudimentary discussion of bonding The structure of 1 has been established using single crystal X - r a y 1 2 8 and neutron diffraction 1 2 9 independently. The N - N bond length is 1.319(4) A , which is intermediate between diazenido and hydrazido activation. If the dinitrogen ligand in 1 is N 2 2 ~ , then the observed diamagnetism suggests that a metal-metal bond exists. D F T studies indicate that there are no occupied molecular orbitals suggestive of a metal-metal b o n d . 1 2 3 1 3 0 Therefore, 1 is better formulated as a tantalum(V) complex with a hydrazido (N24~) ligand. In the formation of 1 from ([NPN|Ta) 2(fx-H) 4 , reducing equivalents are available from the reductive elimination of two bridging hydrido ligands in the tetrahydride precursor as H M . The other two reducing equivalents are supplied by the precursor's metal-metal bond. Since these derived from hydrogenolysis of [ N P N | T a M e 3 , all the reduction of N 2 to N , 4 " in 1 is due to the action of hydrogen gas. It is trivial to prepare 1 i f the hydride precursor is available - the reaction is spontaneous and proceeds in good yield, and no harsh reducing agent is needed. Unlike most highly activated early metal bimetallic dinitrogen complexes, which tend to be linear, the dinitrogen ligand in 1 is asymmetrically activated. Simple charge considerations assign a positive charge to the bridging nitrogen, N b , and a negative charge to the more exposed terminal nitrogen, N, . This latter atom could be drawn as below with Page 19 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design a "lone pair" of electrons pointing away from the T a 2 N 2 core, and a double bond to tantalum. These features are implied in drawings of 1 throughout this thesis. 1.8. Nuclear Magnetic Resonance spectroscopy studies N M R spectroscopy was an important research tool for the work described in this thesis. The solid-state molecular structures of 1 have no symmetry, and at low temperature the solution symmetry is also C, as indicated by the presence of eight si lyl methyl environments in the ' H N M R spectrum. The room temperature solution is C s . with the phosphines, tantalum atoms, and dinitrogen ligand lying on the molecule's plane of symmetry. This is likely due to a "rocking" motion of the [NPN] ligands such that the P-Ta-Ta-P dihedral angle passes through 180°, rendering pairs of s i lyl methyls equivalent and leading to four silyl methyl resonances in the ' H N M R spectrum. The structure of 1 shows that the phosphines are inequivalent, and the 3 1 P { ' H } N M R spectrum of 1 has two resonances at 5 7.8 and 11.0 ppm coupled with J P P = 21.2 H z . 1 2 3 , 1 2 8 The upfield resonance is broadened by rapid nuclear spin relaxation due to the quadrupolar 1 4 N nuclei of the N 2 ligand, which renders the P-P coupling less distinct in Figure 1.4.131 Ph P h Page 20 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design 13^*/*„/-> * 1H NMR spectrum: JC NMR spectrum: ,_ ., , ^ , . .. . t u . . . . Four silyl methyl resonances separate silyl methyl, alkyl A „ , _, • . . . Alkyl and aryl resonances and aryl regions . . . . „_ „ 3 bridging hydrides 5 10.9 ppm Fac/'/e jtt-D enrichment ^ P " 37 P \ N . , . H-. / \ S i M e „ A i ^ ^ - v / /in** M e 2 s ^ Ta Ta «NV M e 2 S i ' l / V^Nr ' ' p h P MW? spectrum: \ p h \ F a c / / e i 5 W enrjchment 6 163.6 (Nt), -20.4(Nb) ppm 1 J N N = 21.5 Hz 29Si NMR spectrum: 6 11.4 ppm, J P S i = 11.3 Hz 5 5.2 ppm, Jpsi = 8.7 Hz S / ^ 2 9 S i 1 = 1/2, 4.68% abundant 11.0 10.0 9D ppm Figure 1.4. Summary of Nuclear Magnetic Resonance spectral data for 1 and its isotopomers. Naturally, the ' H N M R spectrum also reflects C s symmetry, in that only four silyl methyl and four methylene resonances are observed. The bridging hydrides have a distinct chemical shift of 6 10.9 ppm, and the characteristic silyl methyl, methylene, and aryl regions are well separated. The l 3 C { ' H } N M R spectrum of 1 is separated into similar chemical shift regions. 2 9 S i - D E P T N M R spectroscopy of 1 shows two resonances in agreement with C s symmetry, and these resonances are doublets due to coupling to 3 ' P . Because 1 is synthesized from reactions of gases with | N P N | T a M e 3 , the isotopomers (i;NPN|Ta) 2(u J-D) 2((x-ri 1:'n 2-N 2), D2-l, and ([NPN]Ta) 2(u,-H) 2(u,-Ti 1:'r] 2- 1 5N 2), J5N2-l, are readily prepared using isotopically enriched reagents. These isotopomers were instrumental to characterization and mechanistic studies in the research described herein. Page 2/ References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design 1.9. Preliminary reactivity studies The new mode of bonding in ( lP 2 N 2 |Z r ) 2 (^-Ti 2 :T i 2 -N 2 ) enabled new reactions with H 2 and silane at the dinitrogen ligand, as illustrated in Equation 1.12. To explore the possibilities offered by the side-on end-on bonding mode, Johnson performed some experiments with l . 1 2 5 The formation of nitrogen-carbon bonds via migratory insertion with olefins has never been observed in dinitrogen chemistry, and this possibility seemed attractive given the bonding in 1. Unfortunately, the reaction proceeded by another path, as shown in Equation 1.15. (1.15) C H 3 The olefins inserted into the T a - H bonds, resulting in propyl groups at tantalum. In the absence of the bridging hydrido ligands, the dinitrogen moiety isomerized to form a linear bridging complex similar to ( [P 2 N 2 ]Nb) 2 (N 2 ) and ( [P 2 N 2 ]NbMe) 2 (N 2 ) . This implies that the bridging ligands play a significant role in stabilizing the side-on end-on bonding mode of 1. Solution equilibria with Lewis acids such as A l M e 3 have been used to quantify the donor strength of group 6 dinitrogen complexes. 1 3 2 1 3 3 Reactions between 1 and the Lewis acids A l M e 3 , B ( C 6 F , ) 3 , and H B ( Q F 5 ) 2 proceed rapidly and irreversibly to give Lewis acid-base adducts as shown in Equation 1.16.123 Page 22 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design Ph M R 3 = B ( C 6 F 5 ) 3 , H B ( C 6 F 5 ) 2 , A IMe 3 , G a M e 3 1 M R 3 The structures of these complexes show tantalum-dinitrogen cores that are not significantly different from 1 as indicated by bond lengths and angles. The Lewis acidic group 13 metal atoms are trigonal pyramidal, and the N - N and T a - N bonding is conserved. Furthermore, these adducts are stable and the Lewis acids do not dissociate. 1.10. Using molecular orbitals to rationalize reactions The D F T studies mentioned in Section 1.7 led to an understanding of the shapes of some key molecular orbitals of 1. These orbitals can be used to explain the results of the preliminary reactivity studies described in the previous section. A n isosurface of the molecular L U M O is shown in Figure 1.5. 'age 23 References begin on Page 29 Chapter I - Dinitrogen coordination chemistry and reaction design Figure 1.5. Representation of an isosurface of the L U M O of a model complex of 1 as calculated by D F T , and a simplified drawing depicting corresponding metal d orbitals. The L U M O appears to be metal-based. If the jt bond of an incoming olefin interacts with the L U M O , a metal-olefin % complex would result. Subsequent insertion of the olefin into a tantalum hydride bond has ample literature precedent. 1 3 4 Displacement of the bridging hydrido ligands of 1 to non-bridging positions in this manner would allow for isomerization of the hydrazido ligand to the typical end-on, linear mode of bonding. This could explain how the propyl complex shown in Equation 1.15 is formed. The occupied M O s should be more significant to reactions aimed at functionalizing dinitrogen because dinitrogen complexes generally react with electrophiles. The simple drawing of 1 in Equation 1.14 shows a pair of electrons on N, . Equation 1.16 implies they are involved in a dative bond to the Lewis acid when Lewis acid-base adducts are formed. H O M O - 1 , shown in Figure 1.6, has a lobe of electron density in the correct position. Page 24 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design Figure 1.6. Representation of an isosurface of H O M O - 1 of a model complex of 1 as calculated by D F T , and a simplified drawing depicting corresponding metal d and dinitrogen JT* orbitals. This M O is dominated by interactions between the it* molecular orbital of the dinitrogen fragment and d orbitals of suitable orientation on the tantalum atoms. The lobe on N , that is positioned similar to the lone pair drawn on that atom in Equation 1.14 derives from the dinitrogen jt* orbital. This suggests that H O M O - 1 may be involved in dative bonds between 1 and Lewis acids, although a true "lone pair" on N , could be energetically lower than this M O . Negligible change to the N - N bond length was observed in these reactions, which may relate to the observation that H O M O - 1 is based on N-Ta bonding rather than N - N bonding. The H O M O , the M O between the two molecular orbitals already discussed, is shown in Figure 1.7. Page 25 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design Figure 1.7. Representation of an isosurface of the H O M O of a model complex of 1 as calculated by D F T , and a simplified drawing depicting corresponding metal d and dinitrogen jt* orbitals. The H O M O is governed by interactions between the dinitrogen TC* orbital orthogonal to the one involved in H O M O - 1 and d orbitals of appropriate orientation on tantalum. The lobes of the H O M O are symmetric across the molecule's mirror plane, and are almost eclipsed in this representation. It is clear that the T a - N interactions are of TI symmetry. This JT bonding is also implied in the structure drawn in Equation 1.14. The presence of a and jt bonding to the dinitrogen ligand supports the assertion that the metal atoms have reduced the dinitrogen fragment to a hydrazido ligand. The simplified structures shown for H O M O - 1 and H O M O resemble the back-donation interactions shown in Figure 1.3. Scope and prospectus The central goal of this thesis was to develop new reactions for coordinated dinitrogen using the new bonding mode in 1. Secondary, but perhaps more ambitious, goals included exploring processes related to cleavage of the N - N bond and the catalytic incorporation of dinitrogen-derived N atoms into other substrates. Complex 1 offers a highly polarized dinitrogen ligand. It is relatively easy to prepare and purify. It is amenable to a number of N M R spectroscopies, and 1 : , N 2 - and u-D-labeled isotopomers are available. It can react at the dinitrogen ligand, which is nucleophilic. The proposal Page 26 References begin on Page 29 Chapter I - Dinitrogen coordination chemistry and reaction design that the reaction patterns observed for 1 reflect the shapes of certain molecular orbitals suggested that the H O M O should undergo reactions typical of it bonds. Main group hydride reagents react with olefins and a variety of C-heteroatom it bonds via 1,2-addition of the H - E bond across the double bond. Would 1 react with these reagents in a similar manner? The anticipated reaction is illustrated in Scheme 1.6. Scheme 1.6 Because a variety of H - E reagents are available, this scheme proposes functionalization of activated dinitrogen in a way that allows a range of different nitrogen-heteroatom bonds to be made. Since the precursor to 1 is a tantalum hydride complex, the creation of a new tantalum hydride in the same step as N 2 functionalization suggests the possibility of catalytic deployment. The rapid and quantitative reaction between 1 and B ( C 6 F 5 ) 3 to form a Lewis acid-base adduct (Equation 1.16) demonstrated the ease of nitrogen-boron bond formation, and therefore the reactions of 1 with hydroboration reagents were explored. Al though H B ( C 6 F 5 ) 2 rapidly hydroborates olefins, 1 3 3 this boron hydride failed to hydroborate 1. Instead, a Lewis acid-base adduct was formed. This result indicated that the reaction depicted in Scheme 1.6 might not be feasible. However, one of the last experiments in that study gave encouraging results. When 1 is allowed to react with 9-borabicyclononane (9-BBN) , the ' H N M R spectrum of the resulting material has a resonance suggestive of a new terminal tantalum hydrido ligand. Chapter 2 of this thesis describes some hydroboration chemistry of 1. A key finding from this study was that the hydroborated derivatives of 1 are unstable toward elimination of H 2 and subsequent N - N bond cleavage, giving a reactive metal nitrido ligand. Other rearrangements of the resulting complexes are also explored. Chapter 3 Page 27 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design describes hydroalumination of 1 using diisobutylaluminum hydride ( D I B A L ) , a reaction that results in the formation of a new tantalum hydride and a new N - A l bond in a fashion similar to hydroboration. Chapter 4 describes hydrosilylation of 1 using primary and secondary silanes, which proceeds via the same type of intermediate. The loss of H 2 and the N - N bond cleavage event are common to hydroboration, hydroalumination and hydrosilylation. Reactions of the nitrido ligands that arise from this event are also explored. Chapter 5 introduces some surprising results from attempts at hydrozirconation and suggests future directions for this work. Page 28 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design 1.12. References: (1) Greenwood, N . N . ; Earnshaw, A . Chemistry of the Elements; 2nd ed.; Butterworth-Heinemann: Oxford, 1997. (2) Burgess, B . K . Chem. Rev. 1990, 90, 1377. (3) Burgess, B . K . ; Lowe, D. J. Chem. Rev. 1996, 96, 2983. (4) Thorneley, R. N . F.; Lowe, D. J. J. Biol. Inorg. Chem. 1996, 7, 576. (5) Thorneley, R. N . F.; Lowe, D. J. Metal Ions in Biology 1985, 7, 221. (6) Rehder, D . Coord. Chem. Rev. 1999, 182, 297. (7) Pickett, C. J. 1996, 7,601. (8) Einsle, O.; Tezcan, F. A . ; Andrade, S. L . A . ; Schmid, B . ; Yoshida, M . ; Howard, J. B . ; Rees, D. C. Science 2002, 297, 1696. (9) Chan, M . K . ; K i m , .1.; Rees, D. C. Science 1993, 260, 792. (10) Lee, H.-I.; Benton, P. M . C ; Laryukhin, M . ; Igarashi, R. Y . ; Dean, D. R.; Seefeldt, L . C ; Hoffman, B . M . J. Am. Chem. Soc. 2003, 725, 5604. (11) Ertl , G . In Catalytic Ammonia Synthesis; Jennings, J. R., Ed.; Plenum Press: New York, 1991. (12) App l , M . Ammonia; W i l e y - V C H : Weinheim, 1999. (13) Postgate, J. Nitrogen Fixation; Cambridge Press: Cambridge, 1990. (14) Schlogl, R. Angew. Chem., Int. Ed. 2003, 42, 2004. (15) Haber, F. In Nobel Lectures, Including Presentation Speeches and Laureates* Biographies; Frenz, H . , Ed. ; Elsevier Pub. Co. for the Nobel Foundation: Amsterdam, 1918. (16) Bosch, C. In Nobel Lectures, Including Presentation Speeches and Laureates' Biographies; Frenz, H . , Ed.; Elsevier Pub. Co. for the Nobel Foundation: Amsterdam, 1931. (17) Bielawa, H . ; Hinrichsen, O.; Birkner, A . ; Muhler, M . Angew. Chem., Int. Ed. 2001, 40, 1061. (18) Chem. Eng. News 2003, 87, 25. (19) MacKay, B . A . ; Fryzuk, M . D. Chem. Rev. 2004, 104, In press. (20) Lee, S. C ; Holm, R. H . PNAS 2003, 100,3595. (21) Chase, M . W. ; Davies, J. C. A . ; Downe, J. J. R.; Frurip, D. J.; McDonald, R. A . ; Syverue, A . N . J. Phys. Chem. Ref. Data 1985, 14, Suppl. No. 1. (22) Alberty, R. A . J. Biol. Chem. 1994, 269, 7099. (23) Bazhenova, T. A . ; Shilov, A . E. Coord. Chem. Rev. 1995,144, 69. Page 29 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design (24) Mingos, D . M . P. In Comprehensive Organometallic Chemistry; 1st ed.; Wilkinson, G . , Stone, F. G . A . , Abel , E . W. , Eds.; Pergamon Press: Oxford, 1982; V o l . 3 . (25) Aubke, F. ; Wang, C. Coord. Chem. Rev. 1994,137, 483. (26) Al l en , A . D . ; Senoff, C. V . Chem. Commun. 1965, 621. (27) Senoff, C. V . J. Chem. Ed. 1990, 67, 368. (28) Shaver, M . P.; Fryzuk, M . D. Adv. Synth. Catal. 2003, 345, 1061. (29) Hidai, M . M . , Y . Chem. Rev. 1995, 95, 1115. (30) Gambarotta, S. J. Organomet. Chem. 1995, 500, 117. (31) Chatt, J.; Dilworth, J. R.; Richards, R. L . Chem. Rev. 1978, 78, 589. (32) Stoicheff, B . P. Canad. J. Phys. 1954, 82, 630. (33) Sutton, L . E . In Chemical Society Special Publications No. JJ; Chemical Society: London, 1958. (34) Brown, C. J. Acta. Cryst. 1966, 21, 146. (35) Mostad, A . ; Romming, C. Acta Chem. Scand. 1971, 25, 3561. (36) Bouwstra, J. A . ; Schouten, A . ; Kroon, J. Acta Cryst. C. 1983, 39, 1121. (37) Morino, Y . ; Iijima, T.; Murata, Y . Bull. Chem. Soc. Jpn. 1960, 33,46. (38) Col l in , R. L . ; Libscomb, W . N . Acta Cryst. 1951, 4, 10. (39) Fryzuk, M . D. ; Johnson, S. A . Coord. Chem. Rev. 2000, 200-202, 379. (40) Deeth, R. J. J. Organomet. Chem. 2001, 635, 165. (41) Toupadakis, A . ; Kubas, G . J.; King , W . A . ; Scott, B . L . ; Huhmann-Vincent, J. Organometallics 1998, 77, 5315. (42) Fomitchev, D. V . ; Bagley, K . A . ; Coppens, P. J. Am. Chem. Soc. 2000, 122, 532. (43) Goldberg, K . I.; Horrman, D. M . ; Hoffmann, R. Inorg. Chem. 1982, 2J, 3863. (44) Yamabe, T.; Hori , K . ; Minato, T.; Fukui, K . Inorg. Chem. 1980,19. (45) Horn, K . H . ; Lehnert, N . ; Tuczek, F. Inorg. Chem. 2003, 42, 1076. (46) Lehnert, N . ; Tuczek, F. Inorg. Chem. 1999, 38, 1659. (47) Lehnert, N . ; Tuczek, F. Inorg. Chem. 1999, 38, 1671. (48) Basch, H . ; Musaev, D . G . ; Morokuma, K ; Fryzuk, M . D. ; Love, J. B . ; Seidel, W . W. ; Albinati , A . ; Koetzle, T. F.; Klooster, W . T.; Mason, S. A . ; Eckert, J. J. Am. Chem. Soc. 1999,121,523. (49) Chatt, J.; Leigh, G . J. Chem. Soc. Rev. 1972, J, 121. (50) Gao, Y , ; Holah, D. G . ; Hughes, A . N . ; Spivak, G . J. ; Havighurst, M . D . ; Magnuson, V . R. Polyhedron 1998, 17, 3881. (51) Schlaf, M . ; Lough, A . J . ; Morris, R. H . Organometallics 1997,16, 1253. Page 30 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design (52) Jimenez-Tenorio, M ; Jimenez-Tenorio, M . A . ; Puerta, M . C ; Valerga, P. Inorg. Chim. Acta 1997, 259, 77. (53) Komiya, S.; Akita , M ; Yoza , A . ; Kasuga, N . ; Fukuoka, A . ; K a i , Y . J. Chem. Soc, Chem. Commun. 1993, 787. (54) Tsai, Y . - C ; Johnson, M . J. A . ; Mindiola, D . J.; Cummins, C. C ; Klooster, W . T.; Koetzle, T. F. J. Am. Chem. Soc. 1999, 727, 10426. (55) Butts, M . D. ; Bryan, J. C ; Luo, X . L . ; Kubas, G . J. Inorg. Chem. 1997, 36, 3341. (56) King , W . A . ; Scott, B . L . ; Eckert, J . ; Kubas, G . J. Inorg. Chem. 1999, 38, 1069. (57) Manriquez, J. M . ; Bercaw, J. E . J. Am. Chem. Soc. 191 A, 96, 6229. (58) Hidai , M . ; Tominari, K . ; Uchida, Y . J. Am. Chem. Soc. 1972, 94, 110. (59) George, T. A . ; Tisdale, R. C. Inorg. Chem. 1988, 27, 2909.. (60) Trimmel, G . ; Slugovc, C ; Wiede, P.; Mereiter, K . ; Sapunov, V . N . ; Schmid, R.; Kirchner, K . Inorg. Chem. 1997, 36, 1076. (61) George, T. A . ; Seibold, C. D. J. Am. Chem. Soc. 1972, 94, 6859. (62) George, T. A . ; Seibold, C. D. Inorg. Chem. 1973, 72, 2548. (63) Day, V . W. ; George, T. A . ; Iske, S. D. A . J. Am. Chem. Soc. 1974, 97, 4127. (64) Chatt, J.; Elson, C. M . ; Richards, R. L . J. Chem. Soc, Chem. Commun. 1974, 189. (65) Hidai, M . ; Mizobe, Y . Metal Ions in Biological Systems 2002, 39, 121. (66) Leigh, G . J.; Jimenez-Tenorio, M . J. Am. Chem. Soc. 1991, 773, 5862. (67) Smith, J. M . ; Lachicotte, R. J.; Pittard, K . A . ; Cundari, T. R.; Lukat-Rodgers, G . ; Rodgers, K . R.; Holland, P. L . J. Am. Chem. Soc. 2001, 723, 9222. (68) Rocklage, S. M . ; Schrock, R. R. J. Am. Chem. Soc. 1982,104, 3077. (69) O'Donoghue, M . B . ; Davis, W . M . ; Schrock, R. R.; Reiff, W . M . Inorg. Chem. 1999, 38, 243. (70) O'Donoghue, M . B . ; Zanetti, N . C ; Davis, W . M . ; Schrock, R. R. J. Am. Chem. Soc 1997, 779, 2753. (71) O'Donoghue, M . B . ; Davis, W . M . ; Schrock, R. R. Inorg. Chem. 1998, 37, 5149. (72) Ishino, H . ; Kuwata, S.; Ishii, Y . ; Hidai , M . Organometallics 2001, 20, 13. (73) Pool, J. A . ; Lobkovsky, E . ; Chirik, P. J. Organometallics 2003, 22, 2797. (74) Pool, J. A . ; Lobkovsky, E . ; Chirik, P. J. J. Am. Chem. Soc. 2003, 725, 2241. (75) de Wolf, J. M . ; Blaauw, R.; Meetsma, A . ; Teuben, J. H . ; Gyepes, R.; Varga, V . ; Mach, K . ; Veldman, N . ; Spek, A . L . Organometallics 1996, 75, 4977. (76) Chirik, P. J.; Henling, L . M . ; Bercaw, J. E. Organometallics 2001, 20, 534. (77) Caselli, A . ; Solari, E . ; Scopelliti, R.; Floriani, C ; Re, N . ; R izzo l i , C ; Chiesi-V i l l a , A . J. Am. Chem. Soc. 2000, 722, 3652. Page 31 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design (78) Manriquez, J. M . ; McAlister , D. R.; Sanner, R. D . ; Bercaw, J. E. J. Am. Chem. Soc. 1978, 100, 2716. (79) Chatt, J.; Pearman, A . J.; Richards, R. L . J. Chem. Soc, Dalton Trans..1977, 1852. ' (80) Barriere, F. Coord. Chem. Rev. 2003, 236, 71. (81) Chatt, J.; Pearman, A . J.; Richards, R. L . Nature 1975, 253, 39. (82) Leigh, G . J. Acc. Chem. Res. 1992, 25, 177. (83) Nishibayashi, Y . ; Iwai, S.; Hidai , M . J. Am. Chem. Soc. 1998,120, 10559. (84) Nishibayashi, Y . ; Iwai, S.; Hidai , M . Science 1998, 279, 540. (85) Chatt, J.; Diamantis, A . A . ; Heath, G . A . ; Hooper, N . E . ; Leigh, G . J. J. Chem. Soc, Dalton Trans. 1977, 688. (86) Oshita, H . ; Mizobe, Y . ; Hidai, M . J. Organomet. Chem. 1993, 456, 213. (87) Komori , K . ; Oshita, H . ; Mizobe, Y . ; Hidai, M . J. Am. Chem. Soc 1989, 111, 1939. (88) Hori , M . ; M o r i , M . J. Org. Chem. 1995, 60, 1480. (89) M o r i , M . ; Hori , M . ; Akashi , M . ; Hor i , K . ; Sato, M . ; Nishida, M . Angew. Chem., Int. Ed. 1998, 37, 636. (90) Akashi , M . ; Nishida, M . ; Mor i , M . Chem. Lett. 1999, 465. (91) Hori , K . ; M o r i , M . J. Am. Chem. Soc 1998,120, 7651. (92) Ueda, K . ; Sato, Y . ; M o r i , M . J. Am. Chem. Soc. 2000, 722, 10722. (93) Akashi, M . ; Sato, Y . ; Mor i , M . J. Org. Chem. 2001, 66, 7873. (94) Pickett, C. J.; Leigh, G . J. J. Chem. Soc, Chem. Commun. 1981, 1033. (95) Hidai , M . Coord. Chem. Rev. 1999, 185-186, 99. (96) Rees, D . C. Curr. Opin. Struct. Biol. 1993, 3, 921. (97) Laplaza, C. E . ; Cummins, C. C. Science 1995, 268, 861. (98) Laplaza, C. E. ; Johnson, M . J. A . ; Peters, J. ; Odom, A . L . ; K i m , E . ; Cummins, C. C ; George, G . N . ; Pickering, I. J. J. Am. Chem. Soc. 1996,118, 8623. (99) Cu i , Q.; Musaev, D. G . ; Svensson, M . ; Sieber, S.; Morokuma, K . J. Am. Chem. Soc 1995,117, 12366. (100) Groves, J. T.; Takahashi, T. J. Am. Chem. Soc. 1983, 705, 2073. (101) du Bois, J.; Tomooka, C. S.; Hong, J. ; Carrieira, E . M . Acc. Chem. Res. 1997, 30. (102) Maestri, A . G . ; Cherry, K . S.; Toboni, J. J . ; Brown, S. N . J. Am. Chem. Soc. 2001, 123, 7459. (103) Henderickx, H . ; Kwakkenbos, G . ; Peters, A . ; van der Spoel, J . ; de Vries, K . Chem. Commun. 2003, 2050. Page 32 References begin on Page 29 Chapter 1 - Dinitrogen coordination chemistry and reaction design (104) Nugent, W . A . ; Mayer, J . M . Metal-LigandMultiple Bonds; J . Wiley & Sons: New York, 1988. (105) Clentsmith, G . K . B . ; Bates, V . M . E. ; Hitchcock, P. B . ; Cloke, F. G . N . J. Am. Chem. Soc. 1999,727, 10444. (106) Yandulov, D. V . ; Schrock, R. R. J. Am. Chem. Soc. 2002, 124, 6252. (107) Yandulov, D. V . ; Schrock, R. R. Science 2003, 301, 76. (108) Pearson, R. G . J. Chem. Ed. 1968, 45, 581. (109) Fryzuk, M . D. Can. J. Chem. 1992, 70, 2839. (110) Fryzuk, M . D. ; Jafarpour, L . ; Kerton, F. M . ; Love, J. B . ; Patrick, B . O.; Rettig, S. J. In Organometallics, 2001; V o l . 20. (111) Fryzuk, M . D. ; Carter, A . ; Westerhaus, A . Inorg. Chem. 1985, 24, 642. (112) Fryzuk, M . D. ; Haddad, T. S.; Rettig, S. J. J. Am. Chem. Soc. 1990,112, 8185. (113) Fryzuk, M . D. ; Carter, A . ; Rettig, S. J. Organometallics 1992, II, 469. (114) Fryzuk, M . D. ; Love, J. B . ; Rettig, S. J. Chem. Commun. 1996, 2783. (115) Coles, S. J.; Edwards, P. G . ; Fleming, J. S.; Hursthouse, M . B . ; Liyanange, S. S. Chem. Commun. 1996, 293. (116) Caminade, A . - M . ; Majoral, J. P. Chem. Rev. 1994, 94, 1183. (117) Fryzuk, M . D. ; Love, J. B . ; Rettig, S. J. ; Young, V . G . Science 1997, 275, 1445. (118) Fryzuk, M . D. ; Kozak, C. M . ; Bowdridge, M . R.; Patrick, B . 0 . ; Rettig, S. J. J. Am. Chem. Soc. 2002,124, 8389. (119) Fryzuk, M . D. ; Kozak, C . M . ; Patrick, B . O. Inorg. Chim. Acta 2003, 345, 53. (120) Figueroa, J. S.; Cummins, C. C . J. Am. Chem. Soc. 2003, 125, 4020. (121) Mindiola , D. J.; Cummins, C. C. Organometallics 2001, 20, 3626. (122) Bowdridge, M . R., M . Sc. thesis, University of British Columbia, 1998. (123) Johnson, S. A . , Ph. D. thesis, University of British Columbia, 2000. (124) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978,100, 2389. (125) Fryzuk, M . D . ; Johnson, S. A . ; Rettig, S. J. J. Am. Chem. Soc. 2001,123, 1602. (126) Fryzuk, M . D. ; Johnson, S. A . ; Rettig, S. J. Organometallics 2000,19,3931. (127) Pez, G. P.; Apgar, R. K . ; Crissey, J. J. Am. Chem. Soc. 1982,104, 482. (128) Fryzuk, M . D. ; Johnson, S. A . ; Rettig, S. J. J. Am. Chem. Soc. 1998,120, 11024. (129) Fryzuk, M . D. ; Johnson, S. A . ; Patrick, B . O.; Albinati , A . ; Mason, S. A . ; Koetzle, T. F. J. Am. Chem. Soc. 2001,123, 3960. (130) Studt, F. ; MacKay , B . ; Fryzuk, M . D. ; Tuczek, F. J. Am. Chem. Soc. 2004, 126, 3960. Page 33 Rejerences begin on Page 29 Chapter I - Dinitrogen coordination chemistry and reaction design (131) Ebsworth, E . A . ; Rankin, D. W . H . ; Cradock, S. Structural Methods in Inorganic Chemistry; C R C Press: Boston, 1991. (132) Chatt, J.; Crabtree, R. H . ; Jeffery, E . A . ; Richards, R. L . J. Chem. Soc, Dalton Trans. 1973, 1167. (133) Chatt, J.; Crabtree, R. H . ; Richards, R. L . J. Chem. Soc, Chem. Commun. 1972, 9, 534. (134) Doherty, N . M . ; Bercaw, J. E . J. Am. Chem. Soc. 1985,107, 2670. (135) Parks, D. J. ; Piers, W. E . ; Yap, G . P. A . Organometallics 1998,17, 5492. Page 34 Rejerences begin on Page 29 Chapter 2 Hydroboration of coordinated dinitrogen 2.1. Introduction In its simplest terms, the hydroboration reaction is the 1,2-addition of a B - H bond across the it bond of an unsaturated substrate,' although reports of B-a lkyl bond addition have also been referred to as hydroboration. 2 The reactivity of diborane (B 2H,,), its monomeric donor adducts B H , S (where S = ether, thioether, or amine), and its alkylborane derivatives towards double and triple bonds was first developed primarily in the laboratory of H . C. Brown, who was recognized for this work with the Nobel Prize in Chemistry in 1979.3 Diborane is an example of a complex with electron deficient bonding in that the monomeric species dimerizes via bridging H atoms, as shown in Scheme 2.1. disiamylborane thexylborane Scheme 2.1 Page 35 References begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen A review of hydroboration is well beyond the scope of this thesis, but a few general statements regarding hydroboration with alkylboranes are included in this introduction to set the context for the synthetic project described in this Chapter. Alkylboranes can be prepared by hydroboration of alkenes and alkynes with borane, or by the addition of Grignard or alkyllithium reagents to B X 3 (where X is F, CI, or Br) with conventional alkylating agents.4 Primary and secondary boranes are generally dimeric in the solid state with the same bonding as in B 2 H 6 , and therefore ethereal solutions are the most convenient preparations because ethers facilitate dimer dissociation. 3 Other widely used hydroboration reagents include 9 - B B N (9-borabicyclo |3 .3.1 Jnonane), 6 7 dicyclohexylborane, 8 d i s iamylborane , 9 thexylborane 1 0 , ca techolborane" and pinacolborane 1 2 (Scheme 2.1), most of which can be used as purchased from manufacturers. The improved regio- and diastereoselectivity that these reagents offer vs B H 3 S n , which has led to their widespread use, rapid commercialization, 1 3 and ready availability. In contrast to the alkylboranes, the sterically hindered borane (2,4,6-' P r 3 C 6 H 2 ) 2 B H 1 4 is monomeric in solution, and selectively hydroborates alkynes. Hydroborated alkenes and alkynes are easily converted into alcohols, aldehydes, and ketones. 1 3 In cases where the unsaturated substrates are electron poor, the hydroboration reaction needs to be catalyzed, and this is generally done using rhodium complexes. i r > The fact that the hydroboration reaction usually proceeds specifically to give the anti-Markovnikoff, cis addition products with retention of configuration 1 7 makes this technique a key component in modern organic syntheses, and a library of alkylboranes, 1 8 including chiral reagents,19 is currently available. Alkylboranes are also employed as substrates in the Suzuki reaction. 2 0 Hartwig and coworkers have reported alkylborane formation via C - H activation of terminal alkanes using a boryliridium complex, 2 1" 2 3 indicating that alkylborane chemistry continues to be an active and important research area. Unsaturated substrates containing heteroatoms are also amenable to hydroboration. Hydroboration of C = 0 bonds followed by mild oxidative workup was one of the first methods available for the reduction of ketones and aldehydes to alcohols. 2 4 Hydroboration of N - C multiple bonds is also known. Imine hydroboration is generally Page 36 References begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen catalyzed 2 > 2 7 with some exceptions. 2 8 Nitrile hydroboration is a route to monomeric 2 9 and dimeric 3 0 aldiminoboranes, borazines,3 1 and bicyclic B - N - C compounds in low yield. 3 2 2.2. Mechanistic considerations Original proposals for the mechanism of the hydroboration reaction suggested concerted addition via a four-centered transition state with a polarized B - H bond to explain the observed regio- and stereochemistry. 3 3 , 3 4 Brown used G P C analysis of solution samples to follow the kinetics of olefin hydroboration and found that the reaction was first-order in each reagent.3? To rationalize this, Brown suggested that the B 2 H 6 dimer attacked the alkene in a rate-limiting step before releasing a reactive B H 3 monomer to the remaining alkene. A n inverse correlation between rate and steric crowding and a kinetic preference for Z olefins over E were also reported. Later experiments using infrared spectroscopy established that the kinetic order was actually three-halves, which corresponds to a dimer-monomer dissociation equilibrium followed by the slow reaction between monomer and alkene. 1 7 Isotope effects indicate that substantial B - H bond cleavage features in or before the rate-determining step.3 6 Synthetic studies show a correlation between rate of reaction and energy of the alkene H O M O . 3 7 These findings support a putative olefin-borane it complex, which is an interaction between the alkene H O M O and the borane L U M O that is thought to be enhanced by a higher energy H O M O . 3 8 " 4 1 The nature of the monomer-alkene complex was the subject of vigorous debate,4 2"4 4 both before and after the determination of the absolute rate of the reaction of borane with ethylene in the gas p h a s e W e a k l y bound JI complexes were common pre-transition state features of most theoretical proposals, although the exact nature of the transition state itself varied with the level of theory and the olefin substrate. It is now generally accepted that alkene hydroboration proceeds via a four-centered transition state after the formation of a % complex. 4 6 Page 37 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen 2.3. The case for and against hydroboration of dinitrogen A s noted in Chapter 1, dinitrogen does not readily undergo reactions with other substances. Diborane, solutions of borane, and alkylborane are all handled under N 2 , which functions as a protective "inert" gas. Is dinitrogen a substrate that could undergo hydroboration i f it were activated by a transition metal? It was also noted in Chapter 1 that most dinitrogen complexes feature N - N and M - N % bonding, and that most react with electrophiles. In monometallic complexes these JT systems should react with boranes, and the strength of the B - N bond 4 7 may assist thermodynamic considerations. The C O molecule, which is isoelectronic to N 2 , reacts with alkylboranes without transition metal complexation. 4 8 However, there is only one previous report, from Hidai and coworkers, 4 9 of attempts to hydroborate a dinitrogen complex using alkylboranes. In this report, the tungsten(O) dinitrogen complex fra/is-i(dppe) 2 W(N 2 ) 2 | 3 0 was not observed to react with 9 - B B N or dicyclohexylborane. It did react with the triflate of 9 - B B N to give a borylated linear bimetallic dinitrogen complex as shown in Scheme 2.2. O T f P = P ( P h ) 2 v N N = 1 7 0 5 c m - 1 N - N - B = 168 .3 (7 ) ° Scheme 2.2 This complex was structurally characterized, and it was apparent that the dinitrogen complex had attacked the borane, displacing triflate and giving a three-coordinate boron atom. The triflate counterion was also incorporated into the complex, which lost the other N , ligand as is typical of this type of complex in its reactions with anion-electrophile pairs. A n anionic derivative, rr<ms-(NBu 4)[(dppe) 2(NCS)W(N 2)J, Page 38 References begin on Page 70 Chapter 2 - Hydroboration of coordinated dinitrogen reacted sluggishly with 9 - B B N and C y 2 B H , but the H atom introduced with the borane was not observed in the resulting complexes. The anionic complex also reacted with three equivalents of thexylborane to give a different borylated complex (Scheme 2.3). A n external equivalent of thexylborane may remove a boryl proton from an intermediate species, giving a three-coordinate boron atom in the final product. In none of these cases did the authors suggest that B - H bonds were added across the W - N , xt system. [ N B u 4 ] N III N I N C S H 2 B - ^ - ( / N II N I  B H N C S v N N = 1 4 9 5 c m " 1 N - N - B = 1 3 3 . 6 ( 6 ) ° Scheme 2.3 Why should the coordinated dinitrogen moiety resist addition of the B - H bond? A n d why should the B - N bonding in the resulting complexes be so different? It is worth noting that v N N for neutral [(dppe),W(N 2) 2 | is 1953 cm"'," while the anionic complex |(dppe) 2(NCS)W(N 2)]" displays v N N = 1860 cm"'. Therefore, the trans ligand affects the level of N 2 activation, and the two complexes have differing levels of polarization. This means that the boranes in this report were exposed to N 2 moieties with a range of activation levels. That no addition reaction was observed suggests the dinitrogen moiety in these complexes may not be sufficiently activated to participate in hydroboration, which benefits when the unsaturated substrate has a higher energy H O M O . Generally, dinitrogen prefers to bind in an end-on mode even when it is already bound to one metal, and therefore linear bridging complexes of two metals are most common. Many early mechanistic speculations on C-heteroatom hydroboration featured the initial formation of a B-heteroatom adduct followed by H atom transfer. This was based on the observation that rates of hydroboration increased in the presence of ether solvents. 3 2 In Hidai 's report, Page 39 References begin on Rage 70 Chapter 2 - Hydroboration of coordinated dinitrogen it is possible that the basicity of N ( i led to "trapped" Lewis acid-base adducts with four-coordinate boron atoms and that H atom transfer to N a was then unfeasible due to geometric constraints, while external H atom abstraction to give the observed outcomes was favored. Highly activated dinitrogen fragments are characteristic of early metal complexes, and many of these are bridging bimetallic species. The N 2 unit in linear bimetallic N 2 complexes is not usually available to external reagents. The non-linear dinitrogen unit in complex 1 offers an exposed dinitrogen moiety with the high N - N activation typical of early metal systems. This could be a better match for the borane L U M O . The jt H O M O is necessarily of higher energy than H O M O - l , which offers electron density to incoming electrophiles in a a fashion and forms N - M dative bonds. Because of this, the frontier orbitals of 1 offer a way around the potential "trap" of adduct formation in that H O M O -L U M O reactions should be more favorable. The possible outcomes of hydroboration of 1 are also promising. A n y hydroboration of 1 that formed a new N - B bond at the terminal N atom could also form N - H or Ta -H bonds. If T a - H bonds are formed in the process, then regeneration of 1 may be possible and the cycle might be made catalytic. The opportunity to form new tantalum hydrides in the same reaction that functionalizes N 2 is very attractive from this standpoint. A s stated in Chapter 1, preliminary reactivity studies with 1 included reactions with a number of Lewis acids to form adducts of the type shown in Equation 1. The strongly Lewis acidic borane H B ( C 6 F 3 ) 2 hydroborates olefins more rapidly than H B P h 2 or 9 - B B N . 3 3 Curiously, the B - H bond in this reagent does not react with 1. The solid-state molecular structure of [NPN]Ta (L i -H ) 2 (L i -T i 1 : 'n 2 -NNB(H)(C 6 F 5 ) 2 )Ta | ;NPN| 5 4 showed Lewis acid-base adduct formation occurred, rather than B - H bond addition. Page 40 References begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen The reaction in Equation 2.1 is complete in seconds. The side-on end-on bonding mode is preserved in the product complex, and no significant additional N - N or T a - N bond activation is observed despite the electron-withdrawing substituents at B . This suggests that the new dative bond involves electron density that is non-bonding with respect to the dinitrogen fragment. The boron atom in the product is four-coordinate as shown. A broad one-proton resonance typical of a B - H moiety(6 4.70 ppm, F W H M = 250 Hz) is evident in the ' H N M R spectrum, indicating that no H atom transfer has occurred. Several days of heating to reflux in d8-toluene lead to degradation of this adduct without the appearance of resonances typical of either tantalum hydrides or N - H moieties in the ' H N M R spectrum, showing that H atom transfer cannot be forced by heating. Page 41 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen 2.4. Hydroboration of 1 using 9-BBN Johnson's ini t ia l attempts at hydroboration of 1 wi th catecholborane, pinacolborane, and B H 3 T H F adduct led to many products as shown by 3 I P N M R spectroscopy, and hydroboration was nearly abandoned. A s mentioned in Chapter 1, a final trial reaction in that work was the reaction 1 with 9 - B B N to give [NPN|Ta(H)(u.-H X C u - r i ' i r f - N N B Q H J T a C N P N ] , 2.1 (Equation 2.2). The structure proposed for 2.1 was based on its ' H and 3 1 P { ' H } N M R spectra. The 3 1 P { ' H } N M R spectra of 1 and 2.1 each show resonances indicative of inequivalent phosphines, but the appearance in the ' H N M R spectrum of a new one-proton resonance at 6 15.31 ppm and molecular Cx symmetry in solution as shown by the presence of eight discrete si lyl methyl resonances indicated that the H - B bond of 9 - B B N had been added across the T a - N jt bond of 1 to give a new N - B bond and a new terminal hydride as shown in Equation 2.2. M e 2 Ph Ph Ph / T V / " >CA Toluene, T H F M e 2 S i l / \ p h \ 1 P H A 2.1 N ' P h . (2.2) Ph . Ph ft The structure of 2.1 was verified in the solid state by X-ray crystallography at the outset of the work described herein. Figure 2.1 shows an O R T E P depiction of one of the two discrete, enantiomeric molecules present within the asymmetric unit. Metric parameters are very similar for each molecule, and therefore only information for the one shown is reported herein. Selected bond lengths and angles are summarized in Table 2.1. Page 42 Rejerences begin on Page 70 Chapter 2 - Hydroboration of coordinated dinitrogen F i g u r e 2.1. O R T E P drawing (ellipsoids at 50% probabil i ty) , of [NPN]Ta(H ) (L i -H) 2 (u - r 1 , :T i 2 -NNBC x H 1 4 )Ta |NPN| , 2.1 S i l y l methyl and phenyl ring carbons other than ipso omitted for clarity. Hydrides were modeled using X - H Y D E X . Table 2.1. Selected bond distances (A), bond angles (°), and dihedral angles (°) for [NPN|Ta(H ) (L i -H) 2 (u -T 1 1 : r 1 2 -NNBC 8 H 1 4 )Ta |NPN| , 2.1. Atom Atom Distance (A) Atom Atom Distance (A) NI N 2 1.411(15) T a l NI 2.123(12) T a l N2 2.200(12) Ta2 N 2 1.833(11) NI B l 1.40(2) T a l PI 2.583(4) T a l N3 2.076(13) Ta l N 4 2.110(11) Ta2 P2 2.606(4) Ta2 N 5 2.022(11) Ta2 N6 2.019(12) Ta l Ta2 2.8483(8) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) T a l NI N 2 73.9(7) T a l N2 N I 68.0(7) T a l N2 Ta2 89.4(5) Ta l NI B l 163.1(10) NI T a l N3 162.0(5) N3 T a l N 4 95.3(5) N3 T a l PI 81.0(3) PI T a l Ta2 152.17(10) N2 Ta2 P2 163.8(4) N5 Ta2 N6 106.0(5) Page 43 References begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen Atom Atom Atom Atom Dihedral Angle (°) T a l N I N 2 Ta2 6.3(6) PI T a l Ta2 P2 -117.8(2) P2 Ta2 T a l N 2 -174.7(5) P2 Ta2 T a l N3 -4.4(4) P2 Ta2 T a l N 4 106.2(4) B l NI N 2 T a l -168.8(18) The dinitrogen moiety's N - N bond has been elongated from 1.319(4) A in 1 to 1.411(15) A in 2.1. The B - N bond length of the new borylamido unit is 1.40(2) A , the sum of the angles around B is 360.0°, and the sum of angles around N I is 354.6°. The ditantalum-dinitrogen core of the molecule is effectively planar, with an N l - T a l - T a 2 - N 2 torsion angle of -6.3(6)°. Some N - B TC bonding may facilitate this N - N bond elongation since the B atom is very nearly coplanar with the T a 2 N 2 core and is closer to NI than is typical for single bonds. 2 9 The [NPN] ligand bound to T a l appears to be occupying three mutually facial sites with phosphine PI trans to the Ta-Ta axis, amide N3 trans to the dinitrogen fragment and amide N4 cis to N3 at a 95.3(5)° N 3 - T a l - N 4 angle, leaving one face of the Ta atom open. It is likely that the terminal hydride resides on this vacant face. No exchange is observed between the terminal hydride and the two bridging hydrides of 2.1, either on the N M R timescale or on the longer-term chemical timescale.M > The di-u-deuterio isotopomer D2-2.1 (prepared from 1 deuterated at the bridging hydride positions) shows no bridging hydride resonances in its ' H N M R spectrum even after several days. y 7 , : > 8 The side-on end-on bonding mode in 1 has been characterized by ^ N - f H } N M R spectroscopy using J5N2-2.l derived from hydroboration of JSN2-l. The resonance characteristic of the bridging N is at 6 -20.4 ppm, and the terminal N resonance is at 6 163.6 ppm ( 7 N N = 21.5 Hz). The 1 5 N { ' H } N M R spectrum of yW2-2.1 has resonances at 6 -46.9 and 5 24.0 ppm ( '7 N N = 15.3 Hz). The upfield resonance also has a V N P coupling of 25.9 H z and was assigned to the bridging nitrogen atom by comparison to the parent complex. The downfield resonance only displays N - N coupling. Significant chemical shift changes in l 3 N N M R are common when the N atom undergoes a change in hybridization."'9 The decrease in 1 J N N may reflect the elongation of the N - N bond. Page 44 References begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen 2.5. Spontaneous transformations of 2.1 Although stable as a solid, 2.1 is unstable in solution and can be observed by 3 'P -NMR to convert spontaneously into a number of other P-containing species over several days. This capricious reaction is dependent on temperature and solvent. Initial attempts to prepare crystals of 2.1 suitable for X-ray diffraction led to the isolation of a new complex derived from 2.1 by decomposition in solution. The solid-state molecular structure of |NPu J -N]Ta(=NBQH 1 4 )(n-N)(Ta|NPN|, 2.2, is shown in Figure 2.2. Selected bond lengths and angles are summarized in Table 2.2. Figure 2.2. O R T E P drawing of | N P u - N | T a ( = N B Q H , 4 ) ( u . N ) (Ta [NPN| , 2.2 (ellipsoids at 50% probability). Si lyl methyl and phenyl ring carbons other than ipso omitted for clarity. Page 45 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen Table 2.2. Selected bond distances (A), bond angles (°), and dihedral angles (°) for |NP[x-N]Ta(=NBC 8 H 1 4 )( jx-N)(Ta[NPNJ,2.2. Atom Atom Distance (A) Atom Atom Distance (A) T a l N I 1.818(5) NI B l 1.404(9) T a l N 2 2.121(5) Ta2 N 2 1.968(5) Ta2 N3 1.826(5) T a l N3 1.995(5) N 2 S i l 1.717(5) T a l N 4 2.052(5) T a l PI 2.672(2) Ta2 N 5 2.062(5) Ta2 N 6 2.076(5) Ta2 P2 2.683(1) NI N3 2.662(4) T a l Ta2 2.9154(1) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) T a l N2 Ta2 99.5(2) N 2 Ta2 N3 89.0(22) Ta2 N3 T a l 99.5(2) N3 T a l N 2 80.5(2) T a l N I B l 167.0(5) NI T a l P I 102.9(2) N I T a l N 4 111.4(2) Ta l N 2 S i l 125.4(3) N3 Ta2 N 5 116.32 N3 Ta2 N 6 116.3(2) N3 Ta2 P2 93.0(2) Atom Atom Atom Atom Dihedral Angle (°) T a l N I Ta2 N3 -0.5(2) T a l Ta2 N 2 S i l 174.9(6) N I . T a l N 2 Ta2 -110.9(2) PI T a l Ta2 P2 143.1 5(6) Ta2 N2 T a l N4 97.1(3) It is clear that the N - N bond has been completely cleaved during the formation of 2.2. The other rearrangements leading to 2.2 include the loss of the three hydride ligands and one phenyl group from an amido donor. One bridging N atom of 2.2 is bound to a Si atom of a rearranged [NPN] ligand, but the other [NPN] ligand remains unchanged. The borylamido unit of 2.1 has transformed into a borylimido fragment with a T a l - N l bond distance of 1.818(5) A and an essentially unchanged B l - N l distance of 1.404(9) A . Reductive elimination of bridging hydride ligands from 1 as H 2 represents a potential source of two reducing equivalents, however this reaction is not observed even if toluene solutions of 1 are heated to reflux for several days. The square T a 2 N 2 motif in the molecule's core has been observed in dinitrogen cleavage reactions with V 6 0 and N b , M but in both these cases a harsh external reducing agent is employed to achieve N - N Page 46 References begin on Page 70 Chapter 2 - Hydroboration of coordinated dinitrogen bond cleavage. No such reagent is required for the conversion of 2.1 to 2.2. Although 2.1 has bridging hydrides, the solution ' H N M R spectrum of 2.2 shows no evidence for any hydrido ligands. The exclusive observation of D 2 gas and no H D (as observed by G C - M S ) in the headspace above solutions of 2.2 derived from \x-D2-2.\ indicates the reductive elimination of bridging hydrides as H 2 and reinforces the spectroscopic finding that no process leading to exchange between bridging and terminal sites is operant in 2.1. Since parent dinitrogen complex 1 does not exhibit elimination or rearrangement of any kind on its own even after prolonged reflux in toluene, this intricate transformation is triggered by addition of the B - H unit across the Ta -N bond. The conversion of 2.1 to 2.2 is not clean, and competing reactions lead to several side-products as indicated by 3 I P N M R spectroscopy. None of these minor products was isolated. 2.6. 29Si NMR spectroscopy as a probe of mechanism A t first glance, the atoms of the dinitrogen unit of 2.1 apparently correspond to NI of the boryl imide and bridging nitride N3 of 2.2, implying bridging amide N 2 of Figure 2.2 is derived from the ancillary | N P N ] ligand via loss of its phenyl substituent. However, the transformations leading to 2.2 are more complicated. The labeled isotopomer 15N2-\ can be prepared from enriched 1 ; v N 2 and the hydride precursor ([NPN]Ta) 2([A-H) 4 . This allows synthesis of l5N2-2.1 and 15N2-2.2. Whi le l 5 N N M R spectroscopy was not directly helpful in monitoring conversion of 2.1 to 2.2 due to long acquisition times, comparison of the 2 9 S i - D E P T N M R spectra of 2.2 and '*N2-2.2 was very informative. These spectra are shown in Figure 2.3. Page 47 References begin on Page 70 PhJ\ M e 2 S i Chapter 2 - Hydroboration oj coordinated dinitrogen * P h S i M e 2 \ x \ >\ / N \ X \ / 2.2 ~ ~ ^ . S i M e 2 . . .. _ / s i M e 2 N ^ P h P h 1/ 16.0 15.0 14.0 13.0 ppm Figure 2.3. 2 9 S i - D E P T N M R spectra of 2.2 (top) and l5N2-2.2 (bottom). The additional coupling in the 6 13.6 ppm doublet indicates one [NPN] silyl group has migrated onto a dinitrogen-derived N atom. The comparison of these spectra unequivocally establishes that a silicon atom migration occurs during formation of 2.2. Unlabeled 2.2 (top spectrum) shows four doublets (a small two-bond coupling to 3 ' P is generally observed in 2 9 S i N M R spectra of [NPN] ligands), indicating C, symmetry as in the solid state. In J5N2-2.2, the resonance at 6 13.61 ppm is an A M X doublet of doublets consistent with additional coupling to L v N (I = 1/2). Therefore, N 2 of 2.2 (referring to Figure 2.2) originates in the dinitrogen ligand of 1, and the [NPN] silicon atom S i l has migrated to N 2 from an [NPN] ligand amide. This type of migration has been observed in ligands containing N - S i bonds before, 6 2 6 4 leading some researchers to suggest excluding Si from ancil lary l igands . 6 3 , 6 6 Page 48 Rejerences begin on Page 70 Chapter 2 - Hydroboration ojcoordinated dinitrogen Intramolecular shifts of Si atoms from N atoms to O atoms are also known. 6 7 The silyl group migration implied by Figure 2.3 results from competition between N atoms of differing nucleophilicities (amido vs. nitrido). 2.7. Isolation of an intermediate Decomposition of 2.1 to 2.2 can be monitored by 3 1 P N M R spectroscopy. This led to the discovery of an intermediate. Judicious concentration and cooling of a T H F solution of decomposing 2.1 allowed isolation of this material. The solid-state molecular structure of l N P u - N | T a ( = N P h ) ( u - N B ( H ) Q H 1 4 ) T a [ N P N | , 2.3, is shown in Figure 2.4. Selected bond lengths and angles are presented in Table 2.3. Like 2.2, 2.3 shows the square imido-nitrido motif associated with N - N bond cleavage, substantial rearrangement of one | N P N | ligand, and no obvious vacant sites on tantalum that may host hydride ligands. No resonances suggestive of tantalum hydrido ligands are observed in the ' H N M R spectrum of 2.3. Figure 2.4. O R T E P drawing (ellipsoids at 50% probabil i ty) of | N P u - N ] T a ( = N P h ) ( u - N B ( H )C 8 H 1 4 ) T a | N P N | , 2 .3 . S i l y l methyl and phenyl ring carbons other than ipso omitted for clarity. H I was located in the electron difference map and refined isotropically. Page 49 References begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen T a b l e 2 .3 . Selected bond distances (A), bond angles (°), and dihedral angles (°) for [ N P L i - N | T a ( = N P h ) ( L i - N B ( H ) C 8 H 1 4 ) T a [ N P N ] , 2 . 3 . Atom Atom Distance (A) Atom Atom Distance (A) T a l B l 1.92(6) T a l N I 2.184(5) T a l N 2 2.175(5) T a l N3 1.793(5) T a l N 4 2.123(5) T a l PI 2.5900(15) N I ' B l 1.517(8) B l H I 1.33(6) T a l H I 1.92(6) N 2 S i l 1.737(5) NI N 2 2.737(4) Ta2 N 5 2.049(5) Ta2 N6 2.111(5) Ta2 P2 2.7831(15) Atom Atom Atom Angle O Atom Atom Atom Angle (°) NI T a l N 2 77.76(17) T a l N2 Ta2 93.66(18) N2 Ta2 NI 92.3(2) Ta2 N I T a l 96.20(19) T a l N I B l 88.0(3) N I B l H I 101(3) NI T a l N3 113.74(19) NI T A 1 N 4 109.26(18) NI T a l PI 148.57(12) NI Ta2 N 5 101.93(19) N I Ta2 N 6 99.84(19) NI Ta2 P2 167.80(15) Atom Atom Atom Atom Dihedral Angle (°) T a l N2 Ta2 NI -2.7(2) N I T a l N 2 S i l 163.2(3) N 2 T a l N I B l -170.8(3) PI T a l Ta2 P2 346.9(1) Although the square T a 2 N 2 motif in 2 .3 is not significantly different from 2 . 2 , there are two key features of 2 .3 that are not shared with 2 .2. First, the geometry at B l , which is trigonal planar in both 2 .2 and 2 . 1 , is roughly tetrahedral in 2 .3 due to the presence of the bridging hydride H I . A broad one-proton resonance at 6 4.56 ppm in the ' H N M R spectrum of 2 .3 strongly suggests a boron hydride. This hydrogen atom was located in the electron difference map and refined isotropically. H I is potentially involved in an interaction with T a l (Ta l B l = 1.92(6) A , N1-B1-H1 = 101(3)°). The nearly linear T a 2 - N l - B l arrangement may be due to this interaction. It also may indicate a T-shaped hybridization at NI rather than simpler sp 2 or Y-shaped hybridization. 6 8 The second and most significant difference between 2 .2 and 2 . 3 is the linear phenylimido moiety of 2 .3 . A fragment like this would result from the si lyl group migration from an [ N P N | ligand amide to the dinitrogen-derived nitride. After migration, the boron hydride Page 50 References begin on Page 70 • Chapter 2 - Hydroboration oj coordinated dinitrogen and the phenyl group of amide N3 are arranged in a manner that allows for elimination of benzene, a requirement for formation of 2 .2 from 2 .3 . This can be observed via ' H and 1 3 C N M R spectroscopy - both benzene and 2 .2 can be seen in d 8 - T H F solutions of 2 .3 after a few days. Benzene elimination would leave N3 as a terminal nitride ligand. One final question arises: during the formation of 2 .2 , does the terminal nitride attack the opposite tantalum atom, displacing the bridging N - B C 8 H 1 4 moiety to a new position as the terminal and linear imidoborane seen in 2 .2 , or does it simply attack the B C 8 H , 4 group, removing it from the bridging nitride N l in 2 .3? The l : , N labeling study indicates that the former event occurs, and that the B - N bond remains intact throughout these transformations. Compared to the " B spectrum of 2 .3 , the " B spectrum of 1:>N2-2.3 shows a poorly resolved splitting attributed to , : > N - " B coupling. The considerable strength of the B - N bond makes this rearrangement more likely than migration of the borabicyclononyl fragment. Therefore, N l and N 2 in Figures 2 .1, 2.2, and 2.4 are dinitrogen-derived atoms. Most nitrido complexes of group 5 metals tend to form bimetallic squares rather than mononuclear complexes, 6 0 6 8 ' 7 0 suggesting that a new nitrido ligand would prefer to maximize its bonding to transition metals. These transformations are summarized in Scheme 2.4, with the dinitrogen-derived N atoms shown in red throughout. Page 51 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen ti » _ nu Ph Ph . Ph V A iSiMe 2 M e 2 Ph . S i x / p h Ph - ^ r /O-M« 9-BBN . . Ta Ta. / S l M e p M e 2 S i i y)a^—.-- ' d II / \ N >h Toluene, THF N \ ^ Ph' / • \ -N JH, / ^ \ l i ^ H i ^ / \ S i M e 2 7 K Ph - S i r n p ' \ A P H V A > Y > a f . / s i M e 2 H' > V P h V C 6 H Ph* L ft ' N6 t M e 2 •Si Ph . T a Ta- A. S i M e 2 N ^ \ / V V / v P h B Ph 2.1 N ' I B N "Ph Ph — Ph M e 2 / S i ^ / .Ph p \ P h 7 / X N X \ p h ^SiMe 2 T a . / s i M e 2 Ph Ph \ \ . N . pMe 2 •Ta "Ta. /$Me2 2.2 N ^ P h Ph Scheme 2.4 /'age 52 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen 2.8. Further addition reactions A TI bond should exist between Ta and the bridging nitrido ligand in 2.3 based on simple formal charge and hybridization considerations. Even though the bridging nitrido ligand in 2 .2 is derived from an [NPN] ligand, rather than dinitrogen, the possibility of reactions at this atom seemed attractive. It was hoped that this nitrido ligand might react with a second equivalent of 9 - B B N . After 8 hours, the 3 1 P { ' H } N M R spectrum of a solution of 2 .2 and 9 - B B N indicated that a new product had formed. However, no resonances suggestive of either bridging or terminal tantalum hydrides were observed in the ' H N M R spectrum, suggesting that hydroboration had not occurred. Crystals of 2 .4, [ N P ^ - N ] T a ( = N B C 8 H 1 4 ) ( j x - N B ( H ) C 8 H 1 4 ) T a | N P N ] , suitable for X-ray crystallography were obtained from a cooled solution of T H F . The solid-state molecular structure of 2 .4, shown in Figure 2.5, indicates the reaction has proceeded according to Equation 2.3. Selected bond lengths and angles are given in Table 2.4. Page 53 Rejerences begin on Page 70 Chapter 2 - Hydroboration of coordinated dinitrogen F i g u r e 2.5. O R T E P drawing of | N P u - N |Ta( = N - B Q H 1 4 ) ( u - N B ( H ) C 8 H 1 4 ) T a | N P N ] , 2.4 (ellipsoids at 50% probability). S i l y l methyl and phenyl ring carbons other than ipso omitted for clarity. H71 was located in the electron difference map and refined isotropically. Table 2.4. Selected bond distances (A), bond angles (°), and dihedral angles (°) for i N P ^ i - N ] T a ( = N - B C 8 H 1 4 ) ( u - N B ( H ) C x H 1 4 ) T a | N P N | , 2 . 4 Atom A t o m Distance (A) Atom Atom Distance (A) T a i N l 1.824(3) N l B l 1.404(5) T a i N 2 2.175(3) N2 Ta2 1.947(3) Ta2 N3 1.854(3) N3 T a l 2.156(3) N2 S i l 1.738(3) Ta l N 4 2.126(3) T a i PI 2.5825(10) Ta2 N 5 2.091(3) Ta2 N6 2.048(3) Ta2 P2 2.7737(10) B2 N3 1.516(5) Ta l Ta2 3.0153(2) Atom Atom Atom Angle O Atom Atom Atom Angle (°) T a i N2 Ta2 93.87(11) N2 Ta2 N3 91.05(12) Ta2 N3 T a l 97.23(8) N3 T a l N 2 77.58(11) T a i N l B l 179.3(3) N l T a l PI 86.99(10) N l T a i N 4 123.12(13) Ta l N 2 Si4 139.05(16) N3 Ta2 N 5 101.25(13) N3 Ta2 N 6 103.04(12) N3 Ta2 P2 171.01(9) Ta l N3 B 2 88.6(2) Page 54 References begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen Atom Atom Atom Atom Dihedral Angle (°) T a l T a l N I PI Ta2 N 2 N 2 Ta2 T a l T a l N 2 T a l Ta2 N 2 N2 Ta2 T a l NI N3 S i l Ta2 P2 N 4 B2 3.87(11) 6.9(6) -117.54(13) -13.46(4) 98.92(14) -169.5(2) Figure 2.5 shows that the new nitrido ligand has formed a Lewis acid-base adduct with the second equivalent of 9 - B B N rather than undergoing an addition reaction. The coordination environment at B 2 is pseudo-tetrahedral. This is reminiscent of the reaction between 1 and H - B ( Q F 5 ) 2 in that a dative bond has formed without notably affecting the T a - N bonding. It is also similar to 2 . 3 , in that the second equivalent of 9 - B B N in 2.4 occupies a position roughly equal to that of the single 9 - B B N fragment in 2 .3. The T a 2 N 2 core is unaffected by this interaction, which is unsurprising since any "lone pair" on the bridging nitride ligand should be nonbonding with respect to the core by basic symmetry considerations. The borylimido group is essentially unchanged from 2 .2 , except for a slight deflection towards PI that may result from the presence of the second equivalent of 9 - B B N . The ' H N M R spectrum of 2.4 features a broad one-proton resonance at 5 4.56 ppm, which is typical of a boron hydride. Hydride ligand H71 was located in the electron difference map and refined isotropically, and the coordination geometry and location of B 2 suggest that the boron hydride may be interacting with T a l . 2.9. A homologous series of complexes with dicyclohexylborane Hydroboration of 1 represents a completely new reaction for the dinitrogen ligand, and the fact that H B ( C 6 F 5 ) 2 fails to react with 1 in the same manner as 9 - B B N implies that the reaction is influenced by the boryl substituents. For the cascade of reactions shown in Scheme 2.4 to have broad significance to dinitrogen chemistry, the transformations should be general and not dependent on only one fortuitous combination of reagents. Since hydroboration of 1 with 9 - B B N initiates cleavage of dinitrogen, it may be possible and even desirable to attenuate the N 2 cleavage reaction and related steps by Page 55 Re/erences begin on Page 70 Chapter 2 - Hydroboration of coordinated dinitrogen varying the hydroboration reagent. A 1:1 toluene solution of dicyclohexylborane and 1 gives [NPlNGTaCrOCji -^ ix -Ti ' i i i^NNBCQH.^^TalNPN], 2.5, (Equation 2.4). Me2 Ph Ph After stirring overnight, orange 2.5 is available in 93% yield, similar to 2.1. The ' H N M R spectrum of 2.5 indicates Cl solution symmetry (eight silyl methyl resonances and two separate one-proton resonances associated with bridging hydrides are present), and a new resonance at 6 J 6.04 ppm implies the existence of a new terminal hydride, as was found for 2.1. The ' H and 3 1 P N M R spectra of 2.5 are similar to those of 2.1, and therefore it seems other hydroborated species are likely to be attainable by this method. The solid-state molecular structure of 2.5 has not yet been established. Does 2.5 decompose in the same manner as 2.1? Toluene or T H F solutions of 2.5 show conversion to [NPu. -N]Ta(=NPh)(u-NB(H)(C 6 H^ 2 )Ta[NPN] , 2.6, in similar yield and on a similar time scale to the decomposition of 2.1. Therefore, hydroboration and N -N bond cleavage are both possible using C y 2 B H interchangeably with 9 - B B N . The solid-state molecular structure of 2.6 has been determined and it is shown in Figure 2.6. Selected bond lengths and angles are presented in Table 2.5. The relative orientations of the phenylimido ligand and H85 are similar to 2.3. The fact that N - N bond scission and silyl group migration from an [NPN] ligand amide to the new dinitrogen-derived nitrido ligand have occurred in an exactly analogous fashion is evident. Bond lengths and angles are comparable to those of 2.3, except that the [NPN] ancillary ligand bound to Ta2 of complex 2.6 is rotated by 70° about the Ta-Ta axis as compared to its position in 2.3. Page 56 •'References begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen Figure 2.6. O R T E P drawing (ellipsoids at 50% probability) of 2.6, |NPu-N|Ta(=NPh ) (n -NB (H ) (C ( ,H l l ) 2 )Ta |NPN| S i l y l methyl and phenyl ring carbons other than ipso omitted for clarity. H85 was located in the electron difference map and refined isotropically. Table 2.5. Selected bond lengths (A), bond angles (°) , and dihedral angles (°) for INPu-N|Ta(=NPh)(n-N-B(H)(C ( ,H, , ) 2 )Ta|NPN|, 2.6. Atom Atom Distance (A) Atom Atom Distance (A) T a l N l 2.142(3) Ta l N 2 2.191(3) T a l N3 1.788(3) T a l N 4 2.146(3) T a l PI 2.6370(11) N l B l 1.532(7) B l H I 1.61(4) T a l HI 1.89(4) N 2 S i l 1.738(3) N l N 2 2.675(4) Ta2 N 5 2.070(3) Ta2 N 6 2.066(3) Ta2 P2 2.7897(10) Page 57 References begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) NI T a l N 2 76.31(12) T a l N 2 Ta2 94.88(12) N2 Ta2 N I 90.40(13) Ta2 N I T a l 98.41(13) T a l N I B l 89.8(2) NI B l H I 89(3) NI T a l N3 117.17(13) N I T a l N 4 114.08(11) NI T a l PI 136.84(8) N I Ta2 N 5 114.35(12) NI Ta2 N6 117.00(13) N I Ta2 P2 99.46(9) Atom Atom Atom Atom Dihedral Angle (°) T a l N 2 Ta2 N I 0.25(1 1) N I T a l N 2 S i l -176.8(2) N2 T a l NI B l -169.8(2) PI T a l Ta2 P2 -143.96(4) Like 2.3, 2.6 was observed via N M R spectroscopy to eliminate benzene in d 8 -T H F solutions. A t the same time, 3 I P N M R resonances of a new species, 2.7, arise. This species was assigned by ' H and 3 I P N M R spectroscopy as ( |NPu , -N |Ta (=NB(C 6 H u ) 2 ) (u -N)(Ta[NPN|) , 2.7. 2.7 forms cleanly from 2.6 in an overall yield of 83 % by N M R against an internal reference. This is an improvement over the conversion of 2.3 to 2.2, in which many other 3 I P NMR-ac t ive side-products were detected. Although the change from 9 - B B N to dicyclohexylborane did not significantly attenuate hydroboration of 1 or N - N bond cleavage, it did increase the yield of the benzene elimination step. It is also possible that the different geometry of the ancillary [NPN] ligand bound to Ta2 affects the elimination, but testing this hypothesis would be difficult. 2.10. Homologous complexes with thexylborane Since hydroboration of 1 and N - N bond cleavage occurred for the secondary boranes 9 - B B N and C y 2 B H , it was of interest to examine the course of the reactions with a primary borane. The presence of another boron hydride for intramolecular reactions after an initial hydroboration suggested the possibility of different post-hydroboration reaction paths including another potential opportunity to form N - H bonds. The 1:1 reaction between 1 and thexylborane proceeds overnight as shown in Equation 2.5 to give [NPN|Ta(H)(fx-H) 2 (n-ri 1 :Ti 2 -NNB(H)C 6 H 1 3)Ta[NPN|, 2.8, in 92% yield. Page 58 References begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen Me 2 Ph Ph Although crystals suitable for X-ray diffraction have yet to be isolated, the ' H N M R spectrum of 2.8 shows the expected C , symmetry and the new resonance characteristic of a terminal hydride at 5 15.52 ppm. The remaining boron hydride is evident as a broad singlet at 6 4.42 ppm that integrates to one proton. A s expected, solutions of 2.8 are thermally unstable. The rearranged product, 2.9, has no resonances suggestive of bridging or terminal hydride ligands in its ' H N M R spectrum, and, the 3 ' P N M R resonances of 2.9 do not unequivocally make it a sister complex to 2.3 or 2.6. The ' H N M R spectrum is indicative of C, symmetry, and the resonances can be assigned as [NPfx-N]Ta(=NPh)(fx-NB(H) 2 C 6 H 1 3 )Ta |NPN|, as shown in Equation 2.6. Satisfactory elemental analysis was obtained for this formulation, but the solid-state molecular structure of 2.9 has not been established. The only indication that 2.9 might be other than is suggested is that it does not degrade via elimination of benzene. Instead it converts over a week into a number of different products as observed by 3 1 P Page 59 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen N M R spectroscopy. None of these products could be identified as being related to 2.2 or 2.7. Hydroboration of 1 with thexylborane may lead to new rearrangements for 2.8 that were not observed with secondary boranes, but characterization of the resulting complexes has not yet been possible. It is likely that the additional B - H functionality coupled with the already documented propensity for ancillary ligand rearrangements renders this combination too reactive. 2.11. Summary and Conclusions Hydroboration of 1 is possible using a small range of alkylborane hydride reagents that lead to homologous families of complexes as shown in Scheme 2.5. In the hydroboration reaction, two important bonds are formed. Boron-nitrogen bond formation was anticipated based on the known reactions of 1 with B ( C 6 F 5 ) 3 and H B ( Q F 5 ) 2 and a previous example in the literature. 4 9 However, the formation of a terminal tantalum hydride in the same process is a unique feature of this chemistry. This finding is significant because 1 derives from the metal hydride precursor ([NPN|Ta) 2 ([A-H) 4 , and therefore processes that generate new hydride ligands offer the potential for regeneration of 1 in catalytic schemes for nitrogen fixation. Hydroboration with HBL2 Ta[NPN] 1 Adduct formation with ML3 Scheme 2.5 Page 60 Rejerences begin on Page 70 Chapter 2 - Hydroboration ojcoordinated dinitrogen Surprisingly, these hydroborated derivatives all undergo N - N bond cleavage. This event correlates to the loss of bridging hydride ligands as H 2 via reductive elimination, a process that has not been observed for 1 or its Lewis acid adducts (also shown in Scheme 2.5), although it is available in principle to these complexes. Chemical modification via B - H addition across the Ta -N rt bond is necessary to mobilize these additional reducing equivalents and achieve the overall six-electron reduction of the N = N bond. Returning briefly to the theme of catalysis, it is worth noting that all four electrons for reduction of the dinitrogen ligand in 1 to N 2 4 arise from reductive elimination of H 2 : two from the Ta(IV)-Ta(IV) bond of ([NPN|Ta) 2 (u-H) 4 (from hydrogenation of [NPN]TaMe 3 ) and two from reductive elimination of hydrides as H 2 . Therefore the N - N bond cleavage event extends this pattern. Cross-reactivity of reducing agents and electrophiles is a major stumbling block in designing a catalytic cycle for N 2 fixation - the harsh reducing agents used for N 2 complex synthesis as discussed in Section 1.3 would react preferentially with electrophiles rather than with dinitrogen complex precursors, l imiting the amount of activated N 2 available and generating undesirable side products. The role of H , gas as a reducing agent in this system may offer a way around this problem. The nitrido ligands arising from N - N bond scission are reactive, but they irreversibly activate the [NPN] ancillary ligands. This finding does not favor catalytic deployment. It was also found that these new ligands do not undergo additional hydroboration reactions - their reactivity towards alkylborane is limited to Lewis acid-base adduct formation. The changes in hydroboration reagent did not attenuate either hydroboration or the N - N bond cleavage event. However, in the benzene elimination reaction of complexes 2 . 3 and 2.6, the yield of products 2 .2 and 2 .7 (respectively) was higher for the dicyclohexyl derivatives. Mechanistically, the facile hydroboration of 1 presented herein is intriguing in light of the failure of H B ( Q F 5 ) 2 to hydroborate 1. If hydroboration of 1 proceeds via an N-bound intermediate that resembles a Lewis acid-base adduct, then H B ( C 6 F S ) 2 may be too Lewis acidic to transfer the H atom to tantalum or nitrogen once the adduct has formed. Conversely, i f hydroboration of 1 proceeds via a four-centered transition state as is proposed for alkene hydroboration, H B ( C 6 F 5 ) 2 may not be able to form the required jt Page 61 References begin on Page 70 Chapter 2 - Hydroboration of coordinated dinitrogen complex because the molecular dipole of this borane is reversed with respect to typical hydroboration agents.33 This issue remains unresolved. Page 62 Rejerences begin on Page 70 2.12. Experimental Section Chapter 2 - Hydroboration oj coordinated dinitrogen 2.12.1. General Considerations. Unless otherwise stated, all manipulations were performed under an atmosphere of dry oxygen-free dinitrogen by means of standard Schlenk or glovebox techniques (Vacuum Atmospheres HE-553-2 glovebox equipped with a M O - 4 0 - 2 H purification system and a - 60 °C freezer). Anhydrous hexanes and toluene were purchased from A l d r i c h , sparged with dinitrogen, and passed through columns containing activated alumina and Ridox catalyst before use. Anhydrous diethyl ether was stored over sieves and distilled from sodium benzophenone ketyl under argon. Pentane was stored over sieves and distilled from sodium benzophenone ketyl solublized by tetraglyme under dry dinitrogen prior to storage over a potassium mirror. Tetrahydrofuran was heated at reflux over C a H 2 prior to distillation from sodium benzophenone ketyl under argon. Nitrogen gas was dried and deoxygenated by passage through a column containing activated molecular sieves and M n O . Deuterated benzene was dried by heating at reflux with sodium/potassium alloy in a sealed vessel under partial pressure, then trap-to-trap distilled, and freeze-pump-thaw-degassed three times. Deuterated tetrahydrofuran and toluene were dried by refluxing with molten potassium metal in a sealed vessel under vacuum, then trap-to-trap-distilled, and freeze-pump-thaw-degassed three times. Unless otherwise stated, ' H , 3 1 P { ' H } , ' H { 3 1 P } , 1 3 C { ' H } , " B , 1 5 N { ' H } , 2 9 S i - D E P T and two-dimensional N M R spectra were recorded on either a Bruker A M X - 5 0 0 instrument (5mm B B I probe) operating at 500.1 M H z for ' H or a Bruker A V A - 4 0 0 instrument (5mm B B I probe) operating at 400.1 M H z for ' H . ' H N M R spectra were referenced to residual proton in deuterated solvent as follows: C 4 D 7 H O (6 3.58 ppm), Q D 5 H (6 7.15 ppm), and C 7 D 7 H (6 2.09 ppm). 3 1 P N M R spectra were referenced to external P (OMe) 3 (5 141.0 ppm with respect to 85% H 3 P 0 4 at 6 0.0 ppm) or internal P (OMe) 3 flame-sealed inside a 1 mm x 25 mm glass capillary tube if required, l 3 C N M R spectra to l 3 C C 5 D r , (5 128.4 ppm) and 1 3 C D 2 C 1 2 (5 54.0 ppm), " B spectra to neat B F 3 E t 2 0 (5 0.0 ppm), , : > N spectra to external nitromethane at 0.0 ppm, and 2 9 S i to M e 4 S i 50% in C D C 1 3 (5 0.0 ppm). Elemental analyses were performed by M r . P. Borda and M r . M . Lakha, and mass spectrometry (EI /MS on a Page 63 References begmon Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen Kratos M S 50 and G C / M S on a Kratos MS-80) by M r . M . Lapawa, all of the University of British Columbia Department of Chemistry. 2.12.2. Starting Materials and Reagents. 9-borabicyclo|3.3.1]nonane ( 9 - B B N , 1.0 M solution in T H F ) was purchased from Aldr i ch and used without purification. Dicyclohexylborane 8 and thexylborane 1 0 were prepared using literature methods. The synthesis of 1, /5N2-1, D2-l, and their precursors are described elsewhere.7 1 2 9 S i N M R ( Q A , 300 K , 79.5 M H z ) 5 11.36 ppm (d, 2 J P S l 11.3 Hz) 6 5.19 ppm (d, 2 J P S , 8.7 Hz). 2.12.3. Synthesis, Characterization, and Reactivity of Complexes Synthesis of [NPN]Ta(H)(n-H) 2( [i-Ti':Ti 2-NNBC 8H I 4)Ta[NPN], 2.1. To a stirred 20 m L toluene solution of 1 (0.482 g, 0.381 mmol,) was added dropwise 0.40 ml of a 1.0 M solution of 9 - B B N in T H F (used as purchased from Aldrich) in a glovebox. The resulting dark orange mixture was stirred for 24 hours and the toluene was removed in vacuo leaving an orange residue. This residue was rinsed with minimal pentane to afford orange powdery solid 2.1 in 94% yield (0.494 g, 0.358 mmol). ' H N M R ( C 4 D 8 0 , 30 °C, 500 M H z ) : 6 -0.38, -0.24, -0.22, -0.20, 0.09, 0.13, 0.22 and 0.43 (s, 24H total, S i C / / 3 ) , 0.38, 0.96, 1.09, 1.42, 1.49, 1.53, 1.66 and 2.01 ( A M X , 1H each, SiC /7 2 P), 0.31 (br, 1H, N 2 B C 8 # 1 4 ) , 0.60 (br m, 1H, N 2 BC 8 / 7 1 4 ) , 0.81 (br, 1H, N 2 B C 8 # 1 4 ) , 0.98 (br overlapping m, 1H, N 2 B C 8 / / 1 4 ) , 1.12 (br overlapping m, 1H, N 2 B C 8 / / 1 4 ) , 1.18-1.24, and 1.28 (br overlapping m, N 2 BC 8 / 7 I 4 ) 1.36 (br overlapping m, 1H, N 2 B C 8 / / 1 4 ) , 1.56 (br overlapping m, 1H, N 2 B C 8 / / 1 4 ) 6.37, 6.39, 6.48, 6.84, 6.92, 6.98, 7.07, 7.15, 7.27, 7.40, 7.49, 7.53 (overlapping m, P P h - / / and NPh- / / ) 7.77 and 8.15 (m, 2 H each, PPh-o-H), 10.96 (br m, ' H , Ta//Ta) 10.98 (br m, 1H T a M a ) , 15.31 (ddd, 2 7 H P = 74.8 Hz , 7H H= 4 Hz , Jm = 4 Hz , Ta-H,). 1 3 C N M R ( C 4 D 8 0 , 30 °C, 125.76 M H z ) : 0.86, 1.39, 1.60, 1.91, 3.21, 3.60, 4.92 and 5.63 (s or d, S i C H 3 ) , 15.1, 18.5, 19.7, 22.0 (br s, S i C H 2 P ) , 22.9, 26.0, 26.4, 27.7, 32.2 33.0, 35.0 (s, N 2 C 8 H 1 4 ) , 119.7, 121.3, 122.3, 123.5, 126.0, 127.3, 127.6, 128.3, 128.9, 128.9, 129.1, 129.1, 129.5, 129.6, 130.6, 131.9, 134.0, 134.4 (P(C 6 H 5 ) and Page 64 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen N ( C 6 H 5 ) ) . 3 1 P { ' H } N M R (202.4 M H z , C V D 8 , 30 °C): 5 8.3 (d, JPP = 12.0 Hz), 22.7 (d, JPP = 12.0 Hz). 2 9 S i N M R ( C 4 D 8 0 , 30 °C, 79.5 M H z ) 6 10.81, (d, 2 J P S , = 14.78 Hz) , 12.27 (d, 2 J P S , = 9.97 Hz) 13.31 (d, 2 J P S l = 12.72 Hz), 15.71 (d, 2 J P S i = 13.4 Hz) . Ana l . Ca lc 'd for C s 6 H 7 9 B N ( ; P 2 S i 4 T a 2 : C 48.62; H 5.76; N 6.08. Found: C 48.81; H 5.79; N 5.86. Mass Spec (EI/MS) M / z 1383.26 (100%). Synthesis of J5N2-2.l A solution of / j7V2-l was treated in a manner similar to the preparation of 2. 3 1 P { ' H } N M R ( C 6 H 6 , 25 °C): 5 8.3 (dd, 2JPP = 12.0 Hz , V P N = 25.6 Hz), 22.7 (dd, 27 P P = 12.0 Hz , V P N = 2.5 Hz). 1 5 N { ' H } N M R ( C 6 D 6 , 25 °C): 6 -37.7 (dd, 7 N N =14.1 Hz , 2 / N P = 25.6 Hz) , -114.8 (d, lJm= 14.1 Hz). Synthesis of [NP(x-N]Ta(=NBC8HI4)((ji-N)(Ta[NPN], 2.2. A capped 7.5 m L T H F solution of 2.1 (528 mg, 0.382 mmol) was allowed to stand for 6 weeks at room temperature under nitrogen in a glove box. X-ray quality crystals of 2.2 (208 mg, 0.160 mmol, 42% yield) were recovered on a frit after filtration. ' H { 3 1 P } N M R ( C 4 D 8 0 , 300 K , 400 M H z ) : o -0.96, -0.59, -0.41, -0.18, -0.03, 0.04, 0.29, and 0.48 (s, 24H total) S i C / / 3 ; 1.30 and -0.21 (d, 2 J H H = 17.2 Hz), 0.80 and 1.43 (d, 2 J H H = 13.7 Hz), 1.54 and 1.26 (d , 2 J H H = 14.2 Hz) , 1.78 and 1.37 (d, 2 J H H = 24.9 Hz), P C / / 2 (in pairs), I H each; 1.89, 1.93, 1.99, 2.12, complicated overlapping multiplets, 14 H total, B C 8 / / 1 4 ; 6.52 (d, 1H), 6.55 (d, 1H), 6.87(t, 2H), 6.96 (t, 2H), 6.98 (t, 2H), 7.06 (d, 2H), 7.21 (d, 2H), 7.27 (d, 4 H) , N - Q / / 5 ; 7.39 (d, 3H), 7.49 (d, 3H), 8.18 (d, 2H), 8.42 (d, 2H), P- C6H5. I3CVH} N M R ( C 4 D 8 0 , 300 K , 100.61 M H z ) : 6 -3.15, -1.98, -1.06, -1.07, -0.82, 0.02, 0.17, 3.02, S i C H 3 ; 11.34, 17.51, 19.12, 27.92, P C H 2 ; 11.51, 22.76, 32.50, B - C 8 H I 4 (C bound to B not observed); 113.19, 114.23, 119.25, 120.72, 121.27, 126.10, 126.88, 126.94, 127.43, 128.85, 130.97, N - Q H , ; 126.24, 126.94, 128.41, 128.79, 132.01, 133.15, P - C 6 H 5 . 3 1 P { ' H } N M R (161.97 M H z , C 7 D 8 , 300 K ) : 5 -6.5 (d, JPP = 3.96 Hz) , 19.6 (d, JPP = 3.96 Hz). 2 9 S i N M R ( C 4 D 8 0 , 300 K , 79.5 M H z ) 5 12.60 (d, 2 J P s i = 13.41 Hz), 13.61 (d, 2 J P s i = 13.13 Hz), 14.73 (d, 2 J P s , = 11.47 Hz), 16.11 (d, 2 J P i i = 13.96 Hz). Ana l . Ca lc 'd for C 5 0 H 7 1 B N 6 P 2 S i 4 T a 2 : C 46.08; H 5.49; N 6.45. Found: C 46.35; H 5.54; N 6.41. Alternatively a solution of 2.3 was allowed to stand for 4 weeks until its 3 1 P N M R spectrum indicated that the majority of NMR-act ive material was 2.2. This solution was Page 65 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen concentrated by rotary evaporation and allowed to stand in a - 60 °C freezer in a glove box until crystalline 2.2 could be isolated by filtration. Synthesis of D2-2.1 and its decomposition to 2.2. A solution of D2-\ was treated in a manner similar to the preparation of 2.1, giving D2-2.1. No bridging hydride resonances were observed in the ' H { 3 ' P } N M R spectrum of the product, but the downfield terminal hydride resonance was unchanged. The 3 1 P { ' H } spectrum was unchanged except for a slight broadening of the resonances. A thoroughly degassed T H F solution of D2-2.1 was stirred under argon in a sealed vessel for 6 weeks at room temperature and the headspace gas was analyzed by G C / M S , showing D 2 gas and no H D gas. Synthesis of l5N2-2.2. A 3 m L T H F solution of J5N2-2.\ was allowed to stand in a glove box at 15°C. The crystalline material recovered after 6 weeks had a ' H N M R spectrum identical to that of 4 . 3 I P { ' H } N M R ( C 7 D 8 , 300 K , 161.9 M H z ) : 5 -6.5 (d, JPP = 3.96 Hz) , 19.9 (dd, J P N = 5.86 Hz , 7 P P = 3.96 Hz) . I 5 N { ' H } N M R ( C V D 8 , 300 K , 40 M H z ) : -108.2 (s), -230.4 (d, 2 J N P = 5.86 Hz) . 2 9 S i N M R ( C 4 D 8 0 , 300 K , 79.5 M H z ) 5 12.60 (d, 2 J P s i = 13.41 Hz) , 13.61 (dd, 2 J P s , = 13.13 Hz , 'JNSi = 3.2 Hz) , 14.73 (d, 2 J P s i =11.47 Hz) , 16.11 (d, 2 J P s i = 13.96 Hz). Decomposition of 2.1 to [NPji-N]Ta(=NPh)([i-NB(H)C8H14)Ta[NPN], 2.3. A solution of 2.1 in ~5 m L T H F was allowed to stand in a glove box at 15 °C for several days, until the 3 1 P N M R spectrum of a portion of this solution indicated the presence of 2.3 as the major component. The solution was concentrated by rotary evaporation and stored for an additional four days in a - 6 0 ° C freezer, producing X-ray quality crystals of 2.3.. 3 I P { ' H } N M R (161.9 M H z , C 7 D 8 , 300 K ) : 6 -5.79 (d, J P P = 6.7 Hz), 24.51 (d, J P P = 6.7 Hz). ' H { 3 ' P } N M R (400.1 M H z , C 7 D 8 , 300K) 6 -0.78, -0.56, -0.09, -0.05, 0.36, 0.45, 0.46, 0.59, (s, 3 H each, 24H total, S i C / / 3 ) ; -0.85, -0.41, -0.27, -0.04, 0.29, 0.75, 0.93, 1.07 (d, 1H each) PCH2; 1.06, 1.24, 1.35, 1.50, (broad overlapping multiplets, total 14H) B -C 8 / V , 4 ; 4.56 (b, 1H) BH; 6.66, 6.72, 6.75, 6.79, 6.81, 6.92, 6.93, 6.98, 7.05, 7.12, 7.14, 7.17, 7.19, 7.31, 7.49 (doublets, triplets, 20H total) N - C 6 / 7 5 ; 7.59, 7.95, 8.06, 8.22, (10H total) P-C f , / / 5 . Y ie ld was not recorded. Elemental analysis was not obtained. Page 66 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen Synthesis of [NP|i^N]Ta(=N-BC8H14)(|i-NB(H)C8H14)Ta[NPN], 2.4. To a stirred 15 m L toluene solution of 2.2 (298 mg, 0.229 mmol) was added dropwise 0.25 m L (1.1 equiv) of 1.0 M 9 - B B N solution in T H F . The resulting mixture was stirred overnight and the 3 1 P N M R spectrum of a small portion indicated that the reaction had proceeded to consume all of complex 2.2, giving exclusively resonances of 2.4. After evaporation of solvent and precipitation from pentane, 241 mg (0.169 mmol, 74 % yield) of 5 was recovered on a frit. ' H { 3 ' P } N M R (C 6D 6» 300 K , 400 Mhz): 6 -0.69, -0.21, 0.05, 0.13, 0.25, 0.34, 0.43, (s, 3 H each) S i C / / 3 ; 0.84, 0.97 (d, 1H each) P C 7 / 2 , complicated overlapping multipets from 0.70 to 2.26 (total 39 H) B - Q / 7 1 4 and PC7/ 2 (solvent was contaminated with a small amount of hexanes) 4.32 (b, 1H), B-/7; 6.73, 6.79, 6.81, 6.85, 6.93, 7.07, 7.09, 7.20 (s, d, t, some peaks obscured by solvent) Q H 5 - N and C 6 H 5 - P ; ( 7.51 (d, 2 H J H H = 6.95 Hz), 7.94 (d, 2 H .!„,, = 7.05 Hz), o - Q H 5 - P . 3 1 P { ' H } N M R ( Q D 6 , 300 K , 161.9 M H z ) : 6 -12.52 (b), 27.26 (s). Ana l . Ca lc 'd for C 5 8 H 8 6 B 2 N 6 P 2 S i 4 T a 2 : C 48.88; H 6.08; N 5.90. Found: C 49.12; H 6.23; N 5.81. Synthesis of [NPN]Ta(H)(ji-H)2([i-T)':Ti2-NNB(C6H11)2)Ta[NPN], 2.5. Toluene (25 mL) was added to an intimate mixture of dry 1 (0.412 g, 0.327 mmol) and white solid dicyclohexylborane (58.2 mg, 1 equiv) in a glove box. The resulting mixture was stirred vigorously overnight, the solvent was evaporated, and the residues were triturated under hexanes, giving 0.438 mg (0.305 mmol, 93.2 % yield) of 2.5. 3 1 P { ' H } N M R (202.5 M H z , C 6 D 6 , 300 K ) : 6 7.98 (d, J P P = 10.2 Hz) , 20.0 (d, J P P = 10.2 Hz). ' H N M R (500.1 M H z , C 6 D 6 , 300K) 6 -0.37, -0.22, -0.17, -0.05, -0.10, -0.01, 0.09, 0.14 (s, 24H total), SiC77 3; 0.28, 0.40, 0.64, 0.73, 0.77, 0.86, 1.09, 1.15 to 1.34 (broad), 1.47, 1.59, and 1.77 (broad overlapping resonances), cyclohexyl and PCH2; 6.61, 6.66, 6.73, 6.82, 6.88, 6.96, 7.13 to 7.26 (overlapping), 7.29, 7.38, and 7.70 (overlapping doublets, triplets, 20H total, some resonances obscured by solvent) P-C 6 /7 5 and N - C 6 / / 5 ; 8.14 (d, J P H = 6.38 Hz) , 8.24 (d, J P H = 6.82 Hz) , o - P - Q / / 5 ; 10.70 and 11.51 (d, 1H each, J H H = 10.0 Hz) u.-H; 16.04 (d, J P H = 14.6 Hz) , Ta - / / . I 3 C { ' H } N M R was not recorded. Anal . Ca lc 'd for Q o H ^ B N ^ S i J a , : C 50.07; H 6.09; N 5.84. Found: C 50.35; H 6.34; N 5.48. Page 67 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen Decomposition of 2.5 to [NP(i-N]Ta(=NPh)(n-NB(H)(C6H1I)2)Ta[NPN], 2.6. A capped T H F solution of 396 mg (0.275 mmol) 2.5 was left in a glove box at ambient temperature for 3 weeks and then cooled in a - 60 °C freezer, giving crystals of 2.6 Y i e l d 0.186 g, 0.129 mmol, 47%. 3 1 P { ' H } N M R (161.9 M H z , C 7 D 8 , 300 K ) : 6 -2.30 (d, J P P = 3.1 Hz) , 28.30 ( d , J P P = 3.1 Hz). 'H{ 3 1 P} N M R (400.1 M H z , C 7 D 8 , 300K) 6 -1.13, -0.90, -0.09, -0.15, -0.36, 0.18, 0.34 (s, 24H total), S i C / / 3 ; 0.50, 1.16 (d, 1H each) P C / / 2 ; -0.01, 0.68, 0.87, 1.06, 1.33, 1.45, 1.48 (broad overlapping multiplets, total 26H) B - Q / / , , and P C / / 2 ; 4.09 (b, F W H M 32 Hz , 1H) BH; 6.28, 6.30, 6.34, 6.40, 6.50, 6.74, 6.83, 6.94, 7.00, 7.11, 7.18 (overlapping doublets, triplets, 20H total, some resonances obscured by solvent) P - Q / / 5 and N - Q / / 5 ; 7.84 (d, J P H = 6.59 Hz), 7.49 (d, J P H = 7.06 Hz) , (10H total) o -P-C 6 / / 5 . 1 3 C{ 'H> N M R (100.6 M H z , C 7 D 8 , 300K) -1.77, -0.80, -0.64, 0.30, 2.67, 2.75, 3.30, 5.39, S i C H 3 ; 32.85 (b), B - C i p s o of cyclohexyl; 11.14, 15.45, 18.19, 25.93, 26.05, C H 2 of cyclohexyl; 20.99, 25.32, 26.05, 26.41, 27.16, 29.99, 30.49, 31.58, P - C H 2 ; 118.27, 119.03, 120.98, 122.79, 123.16, 123.29, 125.70, 126.98, 127.15, 127.51, 127.74, 128.56, 133.56, 135.84 (some resonances obscured by solvent), P - C 6 H 5 and N - C 6 H 5 ; 127.62, 131.96, o - P - C 6 H 5 ; 151.24, 158.19, i p s o - P - C 6 H 5 . A n a l . C a l c ' d for C 6 0 H 8 5 B N 6 P 2 S i 4 T a 2 : C 50.14; H 5.96; N 5.85. Found: C 50.28; H 6.13; N 5.46. Decomposition of 2.6 to [NPfi-N]Ta(=NB(C(iHll)2)(n-N)(Ta[NPN], 2.7 A d 8 - T H F solution of 2.6 suitable for N M R spectroscopy in a Wilmad N M R tube capped with a plastic stopper and sealed with ParaFilm laboratory f i lm and bearing a sealed capillary containing internal standard was left in a glove box for 3 weeks. Spectra were acquired intermittently. After this time the 3 I P and ' H N M R spectra were exclusively that of 2.7, and the integration with respect to internal standard allowed evaluation of 83% yield. The total integration of other 3 1 P - N M R active resonances was 6%. 'H{ 3 1 P} N M R (400.1 M H z , C 7 D 8 , 300K): 5 -1 .10 , -0.88,-0.34, =0.13, -0.02, 0.18, 0.20, 0.33 (s, 3 H each), S i C / / 3 ; 0.49, 0.75, 0.89, 1.17, 1.21, 1.31, 1.55 (d, 1H each), P-C7/ 2 ; 0.6 to 1.6 (complicated overlapping multiplets, 22H total), B - ( Q / / U ) 2 ; 3.81 (b, F W H M 28 Hz , 1H) B-H; 6.27, 6.34, 6.51, 6.57, 6.63, 6.67, 6.77, 6.83, 6.86, 6.95, 6.99, 7.03, 7.11, 7.16, 7.42, 7.57 (d, t, overlapping, 26H total) P-C6H5and N - C 6 / / 5 ; 7.47, 7.83 (d, 2 H each), o-P-C6H5. Page 68 Rejerences begin on Page 70 Chapter 2 - Hydroboration ojcoordinated dinitrogen 3 1 P { ' H } N M R (161.9 M H z , C 7 D 8 , 300 K ) : 6 0.76 (d, J P P = 5.94 Hz) , 17.41 (d, J P P = 5.94 Hz). Elemental analysis was not obtained. Synthesis of [NPN]Ta(H)([i-H)2(jA-Ti':Ti2-NNB(H)C(;H13)Ta[NPN], 2.8. To a stirred toluene solution of 1 (0.336 g, 0.266 mmol) was added 0.54 ml (0.27 mmol, 1.02 equiv) of freshly prepared thexylborane (0.5 M in T H F ) . After stirring overnight the solvents were evaporated, leaving an orange residue which was triturated under hexanes and recovered on a frit, giving 0.332 g (0.244 mmol, 91.8 % yield) of solid pale orange 2.8. ' H { 3 1 P} N M R (400 M H z , C 6 D 6 , 300 K ) : 6 -0.32, -0.14, -0.05, 0.11, 0.17, 0.19, 0.25, 0.34 (s, 3 H each), S i C / / 3 ; 1.21, 1.41, 1.45, 1.59, 1.61, 1.67, 1.78, 1.81 (d, 1H each) P C / / 2 ; 0.82, 0.89 (d, 3 H each) 1.11, 1.23 (s, 3 H each), B - C ( C / / 3 ) 2 C H ( C / / 3 ) 2 ; 1.64 (m, ' H ) B - C ( C H 3 ) 2 C / / ( C H 3 ) 2 ; 4.42 (b, ' H ) B-H; 5.96, 6.07, 6.22, 6.28, 6.35, 6.66, 6.91, 6.94, 7.01, 7.12, 7.20,7.22, 7.25 (d, t, total 26H), P - Q / / 5 and N - Q / / 5 ; 7.71, 7.92 (d, 2 H each) o - P - C 6 / / 5 ; 10.2, 11.6 (d, ' H each) Tau / / ; 15.52 (s, ' H ) Ta / / . 3 I P { ' H } N M R (161.9 M H z , Q H 6 , 300 K ) : 5 8.73 (d, J P P = 15.3 Hz) , 24.16 (d, J P P = 15.3 Hz). Anal . Calc 'd for C ^ H 7 9 B N 6 P 2 S i 4 T a 2 : C 47.72; H 5.86; N 6.18. Found: C 47.32; H 6.26; N 6.12 Decomposition of 2.8 to [NP|ir-N]Ta(=NPh)(|i-NB(H)2QH13)Ta[NPN], 2.9 A 15 m L toluene solution of 0.328 g (0.241 mmol) 2.8 was allowed to stand in a glove box at ambient temperature for 8 days. The 3 I P N M R spectrum of a portion of this solution showed no remaining 2.8, and solvent was evaporated. Solid 2.9 (134 mg, 41% yield) was recovered on a frit after trituration under hexanes. 'H{ 3 1 P} N M R (400 M H z , C 4 D 8 0 , 300 K ) : 6 -0.48, -0.43, -0.08, -0.02, 0.08, 0.11, 0.27, 0.34 (s, 3 H each, 24H total), S i C / / 3 ; 0.55, 0.78, 1.20, 1.30, 1.57, 1.81, 2.22, 2.36 (d, 1H each), P C / / 2 ; 0.73, 0.81, 1.40, 1.57 (s, 3 H each), B - C ( C / / 3 ) 2 C H ( C / / 3 ) 2 ; 3.58, 4.32 (b, 1H each), B-H; 6.76, 6.80, 6.96, 7.02, 7.05, 7.06, 7.09, 7.12, 7.14, 7.16,7.19, 7.30, 7.33 (d, t, 1 and 2 H each, 26H total) P-C 6 / / 5 and N - C 6 / / 5 ; 7.61, 8.02 (d, 2 H each) P - Q / 7 5 ; 3 I P { ' H } N M R (161.9 M H z , C 4 D 8 0 , 300 K ) : b -6.68 (b) 18.55 (s). Ana l . Calc 'd for C ^ B N ^ S i J a , : C 47.79; H 5.72; N 6.19. Found: C 47.42; H 6.10; N 6.38. Page 69 Rejerences begin on Page 70 2.13. References: Chapter 2 - Hydroboration oj coordinated dinitrogen (1) Brown, H . C. Hydroboration; W . A . Benjamin: New York, 1962. (2) Pelter, A . ; Smith, K . ; Brown, H . C. Borane Reagents; Academic Press: London, 1988. (3) Brown, H . C. Chemistry Nobel Laureate address, 1979; Amsterdam; Elsevier Pub. Co.: New York, 1979. (4) Smith, K . In Organometallics in Synthesis - A Manual; Schlosser, M . , Ed. ; John Wiley & Sons: Chichester, 1994. (5) Brown, H . C ; Kramer, G . W. ; Levy, A . B . ; Midland, M . M . Organic Syntheses via Boranes; Wiley: New York, 1975. (6) Brown, H . C ; Knights, E. F.; Scouten, C. G . J. Am. Chem. Soc. 1974, 96, 7765. . (7) Knights, E. F.; Brown, H . C. J. Am. Chem. Soc. 1968, 90, 5280. (8) Brown, H . C ; Desai, M . C ; Jadhav, P. K . J. Org. Chem. 1982, 47, 5065. (9) Brown, H . C ; Zweifel, G . J. Am. Chem. Soc. 1960, 82, 3222. (10) Zweifel, G . ; Brown, H . C. J. Am. Chem. Soc. 1963, 85, 2066. (11) Brown, H . C ; Gupta, S. K . J. Am. Chem. Soc. 1975, 97, 5249. (12) Tucker, C. E . ; Davidson, J.; Knochel, P. J. Org. Chem. 1992, 57, 3482. (13) Smith, K . Chem. Ind. 1987, 87. (14) Pelter, A . ; Smith, K . ; Buss, D. ; Norbury, A . Tetr. Lett. 1991, 32, 6239. (15) Burgess, K . ; Ohlmeyer, M . J. Chem. Rev. 1991, 97, 1179. (16) Beletskaya, I.; Pelter, A . Tetrahedron 1997, 53, 4957. (17) Chandrasekharan, J.; Brown, H . C. J. Org. Chem. 1985, 50, 518. (18) Ramachandran, P. V . ; Brown, H . C. Organoboranes for Synthesis; Oxford University Press: Washington, D. C , 2001. (19) O'Donnell, M . J. ; Drew, M . D . ; Cooper, J. T.; Delgado, F ; Zhou, C. J. Am. Chem. Soc. 2001,124, 9348. (20) Suzuki, A . In Metal-Catalyzed Cross-Coupling Reactions; Diederich, F. , Stang, P. J., Eds.; W i l e y - V C H : Weinheim, 1998. (21) Chen, H . ; Schlecht, S.; Semple, T. C ; Hartwig, J. F. Science 2000, 287, 5460. (22) Kawamura, K ; Hartwig, J. F. J. Am. Chem. Soc. 2001, 723, 8422. (23) Chen, H . ; Hartwig, J. F. Angew. Chem., Int. Ed. 1999, 38, 3391. (24) Brown, H . C ; Schlesinger, H . I.; Burg, A . B . J. Am. Chem. Soc. 1939, 61, 673. Page 70 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen (25) Vogels, C. M . ; O'Connor, P. E . ; Phillips, T. E. ; Watson, K . J. ; Shaver, M . P.; Hayes, P. G . ; Westcott, S. A . Can. J. Chem. 2001, 79, 1898. (26) Baker, R. T.; Calabrese, J. C ; Westcott, S. A . J. Organomet. Chem. 1995, 498, 109. (27) Buranov, A . U . ; Morr i l l , T. C. Tetr. Lett. 2003, 44, 6301. (28) Montury, M . ; Gore, J. Bull. Soc. Chim. France 1975, 2622. (29) Yalpani, M . ; Koester, R.; Boese, R. Chem. Ber. 1993,126, 285. (30) Yalpani, M . ; Koester, R.; Boese, R. Chem. Ber. 1992,125, 15. (31) Jennings, J. R.; Wade, K . J. Chem. Soc, A 1968, 1946. (32) Coult, R.; Fox, M . A . ; Rand, B . ; Wade, K ; Westwood, A . V . K . J. Chem. Soc, Dalton Trans. 1997, 3411. (33) Brown, H . C ; Zweifel, G . J. Am. Chem. Soc. 1960, 82, 4708. (34) Brown, H . C. J. Am. Chem. Soc. 1961, 83, 2544. (35) Brown, H . C ; Moerikoffer, W. J. Am. Chem. Soc. 1961, 83, 3417. (36) Pasto, D. J. ; Lepeska, B . J. Am. Chem. Soc. 1976, 98, 1091. (37) Nelson, D. J. ; Cooper, P. J. Tetr. Lett. 1986, 27, 4693. (38) Clark, T.; Schleyer, P. v. R. J. Organomet. Chem. 1978,156, 191. (39) Dasgupta, S.; Datta, M . K . ; Datta, R. Tetr. Lett. 1978, 1309. (40) Dewar, M . J. S.; McKee , M . L . Inorg. Chem. 1978, 77, 1075. (41) Sundberg, K . R.; Graham, G . D.; Lipscomb, W. N . J. Am. Chem. Soc. 1979,101, 2863. (42) Streitwiesser, A . , Jr.; Verbit, L . ; Bittman, R. J. Org. Chem. 1967, 32, 1530. (43) Pasto, D. J.; Kang, S.-Z. J. Am. Chem. Soc 1968, 90, 3797. (44) Nagase, S.; Ray, N . K . ; Morokuma, K . J. Am. Chem. Soc. 1980,102,4536. (45) Fehlner, T. P. J. Am. Chem. Soc 1971, 93, 6366. (46) Wang, X . ; L i , Y . ; Wu , Y . D . ; Paddon-Row, M . N . ; Rondan, N . G . ; Houk, K . N . J. Org. Chem. 1990, 55, 2601. (47) Encyclopedia of Inorganic Chemistry; K ing , R. B . , Ed.; Wiley: Chichester, 1994. (48) Brown, H . C. Acc Chem. Res. 1969, 2, 65. (49) Ishino, H . ; Ishii, Y . ; Hidai, M . Chem. Lett. 1998, 677. (50) Hidai , M . M . , Y . Chem. Rev. 1995, 95, 1115. (51) Be l l , B . ; Chatt, J . ; Leigh, G . J. J. Chem. Soc, Dalton Trans. 1972, 2074. (52) Brown, H . C ; Subba Rao, B . C. J. Am. Chem. Soc 1956, 78, 5694. (53) Parks, D. J. ; Piers, W. E. ; Yap, G . P. A . Organometallics 1998, 77, 5492. Page 71 Rejerences begin on Page 70 Chapter 2 - Hydroboration oj coordinated dinitrogen (54) Johnson, S. A . , Ph.D. thesis, University of British Columbia, 2000. (55) Orpen, A . G . J. Chem. Soc, Dalton Trans. 1980, 2509. (56) Fryzuk, M . D.; McConvi l le , D. H . Inorg. Chem. 1989, 28, 1613. (57) McCleverty, J. A . ; Wilkinson, G . Chem. Ind. 1961, 288. (58) Antinolo, A . ; Carrillo-Hermosilla, F. ; Fajardo, M . ; Fernandez-Baeza, J. ; Garcia-Yuste, S.; Otero, A . Coord. Chem. Rev. 1999, 43. (59) Mason, J. Chem. Rev. 1981, 81, 205. (60) Clentsmith, G . K . B . ; Bates, V . M . E. ; Hitchcock, P. B . ; Cloke, F. G . N . J. Am. Chem. Soc. 1999, 121, 10444. (61) Kawaguchi, H . ; Matsuo, T. Angew. Chem., Int. Ed. 2002, 41, 2792. (62) Fryzuk, M . D. ; MacNei l l , P. A . j. Am. Chem. Soc. 1984,106, 6993. (63) Gountchev, T. 1.; Ti l ley, T. D . J. Am. Chem. Soc. 1997,119, 12831. (64) Herrmann, W . A . ; Baratta, W . J. Organomet. Chem. 1996, 506, 357. (65) Schrock, R. R. Acc Chem. Res. 1997, 30, 9. (66) Freundlich, J. S.; Schrock, R. R.; Davis, W . M . J. Am. Chem. Soc. 1996, 778, 3643. (67) Abe, M . ; Shirodai, Y . ; Nojima, M . J. Chem. Soc, Perkin Trans. 1998, 79, 3253. (68) Nugent, W . A . ; Mayer, J. M . Metal-LigandMultiple Bonds; J. Wiley & Sons: New York, 1988. (69) Fryzuk, M . D.; Johnson, S. A . Coord. Chem. Rev. 2000, 200-202, 379. (70) Caselli, A . ; Solari, E . ; Scopelliti, R.; Floriani, C ; Re, N . ; R izzo l i , C ; Chiesi-V i l l a , A . J. Am. Chem. Soc. 2000, 722,3652. (71) Fryzuk, M . D.; Johnson, S. A . ; Patrick, B . O.; Albinati , A . ; Mason, S. A . ; Koetzle, T. F. J. Am. Chem. Soc. 2001, 723, 3960. Page 72 References begin on Page 70 Chapter 3 Hydroalumination of coordinated dinitrogen 3.1 Introduction The preceding chapter describes the hydroboration of 1, an unprecedented reaction for coordinated dinitrogen. The surprising chain of events following the addition reaction was also introduced. After 1 is hydroborated with primary and secondary alkylboranes, reductive elimination of bridging hydrido ligands as H 2 and N - N bond cleavage consistently occurrs, and a correlation between these events is suggested. In general, the hydroboration reactions proceed over hours at typical reaction conditions. However, the subsequent processes occur over days or weeks. Although the yields of the hydroborated species 2.1, 2.6, and 2.8 were excellent, yields of complexes displaying N -N bond cleavage were poor or fair due to competing reactions. Irreversible modification of the ancillary | N P N ] ligand by the nitrido ligand generated by the N - N bond cleavage process also occurs. The work described in this chapter explores hydroalumination of 1 with organoaluminum reagents. It was hoped that the different properties of aluminum would lead to other reaction pathways for derivatives of 1. Boron and aluminum have related but distinct chemistries,1 as do their organometallic compounds. 2 A luminum readily transfers alkyl groups to unsaturated substrates in a reaction known as "carboalumination". 3 5 Early developments in the synthesis of organolithium, -magnesium, and -boron reagents benefited from the common practice of using ether solvents. In hindsight, this habit had a negative effect on organoaluminum reagents because their ether adducts are less reactive. Page 73 Rejerences begin on Page 96 Chapter 3 - Hydroalumination oj coordinated dinitrogen Organoaluminum reagents are sufficiently Lewis acidic that ether "retarded not only carboalumination but, indeed, the whole development of organoaluminum chemistry." 6 Aluminum hydride or alane, A 1 H 3 , is more ionic than borane, and exists as a three-dimensional polymer in the solid state rather than as a dimer. 7 8 B H 3 S and A 1 H 3 S (where S is an ether donor) 9 add across C=C bonds and C-heteroatom double bonds, and the mechanisms for these reactions are thought to have common features.10"12 Boron has an extensive covalent cluster chemistry because hydrides of boron readily self-associate,13 while cluster chemistry of aluminum is dependent on reduction by alkali metals' 4 and availability of the Al ( l ) oxidation state.' 3 1 6 The best-known organoaluminum compounds are A l M e 3 and A l E t 3 , which self-associate as dimeric A 1 2 R 6 species with structures similar to B 2 H 6 . These organometallic compounds are widely utilized as Lewis acid catalysts in synthesis and cocatalysts in polymerization. A l M e 3 is known to react with 1 to form a stable Lewis acid-base adduct as discussed in Chapter 1. The synthesis of specific alkylaluminum and alkylaluminum hydride reagents can be more complicated than organoboron synthesis in that the hydroalumination reaction is more easily reversible than hydroboration via facile p elimination. 1 7 The extent to which this happens depends on the relative stabilities of R n AlH 3 .„, which vary with the alkyl substituents.'8 When olefin hydroalumination does not give the desired organometallic reagent, direct synthesis is approached via a Grignard reaction with M X 3 (where X is generally CI) . ' 9 Conproportionation of R 3 A1 and A 1 X 3 can also be used. 2 0 The resulting R 2A1C1 and RA1C1, species generally dimerize but are available as monomers when the R group is sufficiently bulky. 2 ' Because of the ease with which the intermediates and products redistribute, organoaluminum halides must be reduced with aluminum-free reducing agents such as L i H to give good yields. 2 2 The tendency of aluminum reagents to exchange groups can lead to rearrangement in hydroalumination reactions of olefins. Consequently, lower stereo- and regiospecificity are observed with hydroalumination than with hydroboration, and therefore these reagents are less suitable and less developed for this type of reaction than their boryl congeners.6 The widely employed hydride reagent diisobutylaluminum hydride, D I B A L , is formed commercially from triisobutylaluminum , A l ( s ) , and H 2 , or by combination of A l ( S i ) , H 2 , and isobutylene. 1 0 Upon heating, triisobutylaluminum gives diisobutylaluminum Page 74 References begin on Page 96 Chapter 3 - Hydroalumination oj coordinated dinitrogen hydride and isobutylene by (3 elimination. 2 3 D I B A L is generally used in organic synthesis for the reduction of ketones and aldehydes. Gas-phase evidence indicates a four-centered transition state in olefin hydroalumination with D I B A L . 2 4 The structural similarity of D I B A L to 9 - B B N suggested its use in this study. 3.2 Hydroalumination of 1 The reaction of a D I B A L solution in hexanes with a toluene solution of 1 is immediate as shown by the distinct color change from red-brown to yellow-brown. Both the immediacy of this reaction and the color change are reminiscent of the reaction between 1 and A l M e 3 , which forms a stable Lewis acid-base adduct as was shown in Chapter 1 (Equation 1.16 on Page 23). This initially suggested that the reaction of 1 with D I B A L was simply formation of the dative N - A l bond in a Lewis acid-base adduct, rather than the desired addition reaction, implying that the A l atom in the product would be tetracoordinate. The 3 I P N M R spectrum of the new species, complex 3.1, shows two new resonances, 6 12.1 and 14.5 ppm, coupled with J P P = 18.3 Hz . The chemical shifts and line shapes, especially the broadness of the downfield resonance, are reminiscent of the 3 1 P N M R spectrum of 1, as shown in Figure 3.1. These resonances are not suggestive of a Lewis acid-base adduct of 1, nor do they suggest that 3.1 is related to hydroborated complexes 2.1, 2.5, and 2.8 from Chapter 2, which have very similar 3 I P N M R spectra. The spectrum of 3.1 shown was acquired immediately after the addition of one equivalent of D I B A L to the N M R sample used to obtain the spectrum of 1, demonstrating that the reaction is complete before the sample can be returned to the spectrometer. Product 3.1 is unstable and converts completely to a different product in solution over the course of five hours at room temperature as observed by 3 1 P N M R spectroscopy. This conversion implies the first product, complex 3.1, is distinct from the Lewis acid adducts of 1, which are reasonably stable in solution and do not degrade unless heated. Page 75 Rejerences begin on Page 96 Chapter 3 - Hydroalumination ojcoordinated dinitrogen 1 • 1 • —r • 1 • 1 • r — — • 1 • 1 • 1 1 6 1 4 1 2 1 0 8 6 4 2 p p m Figure 3.1. 3 1 P N M R spectrum of 1 (bottom) prior to addition of D I B A L . Conversion to complex 3.1 (top) occurs rapidly. The solid-state molecular structure of 3.1 has not been established, but it was characterized by N M R spectroscopy at low temperature. The ' H { 3 I P } N M R spectra of 3.1 obtained at 300 K and at 213 K each show eight silyl methyl resonances and a new one proton resonance at 5 19.2 ppm, indicative of a terminal tantalum hydride. Resonances for isobutyl groups are also observed, indicating incorporation of the dialkylaluminum moiety into 3.1. These findings suggest that the A l - H bond of D I B A L has added across the T a - N it bond of 1 to give C, molecular symmetry, a new N - A l bond and a new terminal tantalum hydride (6 19.2 ppm) as in the hydroboration of 1. Hydroalumination of 1 to give [NPN|Ta(H)(n-H) 2(n-r 1 1:'ri 2-NNAl'Bu 2)Ta|;NPN:] is shown in Equation 3.1. M e 2 P h P h Page 76 Rejerences begin on Page 96 Chapter 3 - Hydroalumination of coordinated dinitrogen The fate of the dinitrogen moiety in 3.1 was probed by low-temperature ^N-J'H} N M R spectroscopy of ,5N2-3.l, prepared from "N2-l and D I B A L . The 1 5 N { ' H } N M R spectrum of J5N2-3.1 is similar to " A ^ - l , in that it features two resonances coupled to each other by 18.7 Hz , implying an intact N - N bond. The bridging N resonance at 6 -26.2 ppm shows couplings of J P N = 26.0 H z and J N N = 18.7 Hz , and the terminal N resonance at S -32.9 ppm shows couplings of J P N = 6.5 Hz and J N N = 18.7 Hz. This is a marked change in chemical shift from 6 163.6 ppm in " A ^ - l . A similar dramatic change was seen for the terminal N resonance in conversion of l5N2-\ to '5N2-2.\. The J N N coupling, similar to the coupling constants reported for l5N2-l and l5N2-2.\, implies the N - N bond is intact. 3.3 Exploring the transformations of 3.1 using31P NMR spectroscopy The fact that 3.1 undergoes transformations in solution suggested that a time-resolved 3 1 P N M R study of the ensuing reactions would be informative. Figure 3.2 shows a stack-plot of 3 I P { ' H } spectra acquired every 60 minutes, representing ten hours of a reaction pathway that begins with formation of 3.1, proceeds to an intermediate (3.2), and terminates in two products (3.3 and 3.4). The species in this reaction pathway all have two 3 ' P N M R spectral resonances, but in the interests of clarity the spectral width in Figure 3.2 was selected to display only one resonance of each species. Fage 77 References begin on Page 96 Chapter 3 - Hydroalumination of a ordinated dinitrogen 20 19 18 17 16 15 14 13 12 11 ppm Figure 3.2. 3 1 P N M R spectra ( C 6 D 6 , 300K) at hourly intervals show the conversion of 3.1 (blue) into 3.2 (red), which converts in turn to 3.3 and 3.4 (yellow and green). This experiment was performed with an internal " P reference for integration. The total integration of 3 1 P resonances vs internal standard is constant for all spectra. This indicates a lack of NMR-inact ive or paramagnetic intermediates in the course of these reactions. The integral values for the species of interest are plotted vs time in Chart 3.1. Page 78 References begin on Page 96 Chapter 3 - Hydroalumination of coordinated dinitrogen The clear implication of Chart 3.1 is that 3.1 (dark blue) converts completely over 5 hours to a new species, 3.2 (magenta). After 6 hours, the resonances of 3.2 begin to decrease and species 3.3 and 3.4 (yellow and turquoise, respectively) arise concurrently. Complexes 3.3 and 3.4 can be considered as the end-points of this process because they do not undergo further rearrangements. Scheme 3.1 summarizes these findings. Scheme 3.1 Kinetic analysis of this data wi l l be discussed briefly in a later section, following the presentation of information pertaining to the composition and formation of complexes 3.2, 3.3, and 3.4. Page 79 References he gin on Page 96 Chapter 3 - Hydroalumination ojcoordinated dinitrogen 3.4 Molecular structure of 3.3 End-product 3.3 crystallizes from hexanes, and the solid-state molecular structure of 3 .3 has been obtained using X-ray crystallography. A n O R T E P drawing of 3 .3 is shown in Figure 3.3. Figure 3.3. O R T E P drawing of 3.3 (ellipsoids at 50% probability). S i ly l methyl and phenyl ring carbons other than ipso omitted for clarity. H72 and H73 were located in the difference map and refined isotropically. Clearly, N - N bond cleavage has occurred, as shown by the N l N 2 distance of 2.657(3) A. The T a 2 N 2 square is like several compounds described in Chapter 2. Although the [NPN1 ligands are still intact, one | N P N | ligand amido donor has migrated from T a l to A l l , which is still bound to N l . It is also significant that the A l atom has lost one of its isobutyl groups. Selected bond distances and angles are summarized in Table i 1 3.1, which appears later in this Chapter. A drawing of | N P N - A l ( H ) C 4 H 9 | T a ( u - N ) ( u -I'age HO References begin on Page 96 Chapter 3 - Hydroalumination Ojcoordinated dinitrogen N ) T a | N P N | , 3 . 3 based on formal charge considerations shows a highly ionic interpretation of this structure (below). Both tantalum atoms and the aluminum atom are in their highest oxidation states. Two dinitrogen-derived N 3 " nitrido ligands bridge the tantalum atoms, forming a typical T a 2 N 2 square. The aluminum atom is coordinated by one [ N P N | ligand amide, one isobutyl substituent, one hydrido ligand, and one bridging nitrido ligand. A proximal pair of hydrido ligands remains in 3 .3 , both of which were located in the electron difference map and refined isotropically. One terminal tantalum hydride is suggested by the presence of a doublet at 6 17.36 ppm with J H H = 11.3 Hz in the 'H{ 3 1 P} N M R spectrum of 3.3. In the absence of the solid-state molecular structure, the source of the coupling causing this doublet splitting was not initially obvious. N - N bond cleavage is apparent in Figure 3.3 and is suggested by a lack of scalar coupling between resonances in the l v N { ' H } N M R spectrum of l5N2-3.3. This event implies elimination of bridging hydrides, and no resonances suggestive of any other tantalum hydrido ligands in either bridging or terminal positions appear in the ' H N M R spectrum. The ' H / ' H C O S Y spectrum of 3.3 shows a cross-peak between the terminal hydride and a broad one-proton resonance at 6 1.23 ppm (doublet, J H H = 11.3 Hz) , which is hidden among methylene resonances of the | N P N | and isobutyl ligands of 3 .3 in the one-dimensional ' H N M R spectrum. These two hydrido ligands are held in close proximity by the tantalum and aluminum atoms bound to N l . The H 7 2 - T a l - A l l - H 7 3 dihedral angle is nearly 90° , and therefore the magnitude of the coupling constant between these sites suggests geminal rather than vicinal coupl ing , 2 3 2 6 In turn, this suggests that a T a - A l bond is not likely. The P h 3.3 Page 81 Rejerences begin on Page 96 Chapter 3 - Hydroalumination of coordinated dinitrogen Ta l A l l distance is 2.9275(15) A, substantially longer than seen in a previously reported complex featuring a dative T a - A l bond, bridging hydride, and terminal aluminum hydride. 2 7 Formal charge considerations in 3.3 do not require a T a - A l bond. 3.5 Complexes 3.3 and 3.4 are diastereomers The other ultimate product of the decomposition of 3 .1 , complex 3.4, was crystallized independently of 3.3. The solid-state molecular structure of 3.4 shows that it is very similar to 3 .3 even though the " P { ' H } N M R spectra of 3 . 3 and 3.4 are very different in terms of chemical shift. A n O R T E P drawing of 3.4 is shown in Figure 3.4. Figure 3.4. O R T E P drawing of the solid-state molecular structure of 3.4 (ellipsoids at 50% probability). Si ly l methyl and phenyl ring carbons other than ipso omitted for clarity. H72 and H73 were modeled using X - H Y D E X . 2 8 It is clear from comparison of Figure 3.3 to Figure 3.4 that the difference between 3.3 and 3.4 is the rotational orientation of the | N P N | ligand bound to Ta2. In 3.3 the P-Ta-Ta-P dihedral angle is -115 .32° whereas the same angle in 3.4 is 79.94°. Otherwise, the compositions and connectivities of 3.3 and 3.4 are identical. Selected bond lengths Page 82 References begin on Page 96 Chapter 3 - Hydroalumination of coordinated dinitrogen and angles for the two complexes are compared in Table 3.1, and the experimental details of the diffraction experiments are tabulated in Appendix 1. 3.3 is referred to as anti and 3.4 as syn based on relative orientations of P2 and A l l . Table 3 .1 . Comparison of selected bond distances (A), bond angles (°), and dihedral angles (°) for complexes 3.3 and 3.4. Atom Atom Distance (A) in 3.3 Distance (A) in 3.4 T a l N l 2.158(4) 2.098(3) T a l N2 1.861(4) 1.875(3) Ta2 N l 1.892(4) 1.918(3) Ta2 N 2 1.917(4) 1.907(3) NI N2 2.657(3) 2.606(4) T a l Ta2 2.8737(3) 2.84043(19) T a l PI 2.6275(13) 2.6316(9) T a l N4 2.037(4) 2.054(3) T a l A l l 2.9275(15) 2.9105(10) Ta2 P2 2.7423(13) 2.7273(9) Ta2 N 5 2.053(4) 2.039(3) Ta2 N6 2.049(4) 2.046(3) NI A l l 1.856(4) 1.856(3) A l l N3 1.855(4) 1.845(3) Atom Atom Atom Angle (°) in 3.3 Angle (°) in 3.4 T a l N l Ta2 90.15(16) 89.92(11) T a l N 2 Ta2 99.03(17) 97.34(13) N l T a l N 2 82.38(16) 83.56(11) N 2 Ta2 N I 88.43(17) 87.80(12) T a l N l A l l 93.34(17) 94.60(12) Ta2 N l A l l 165.0(2) 156.79(17) N 4 T a l PI 77.07(11) 74.22(8) N l A l l N3 115.25(19) 117.97(14) PI T a l N l 88.66(10) 90.24(8) N l T a l N 4 165.73(14) 161.03(11) N 5 Ta2 N6 116.10(17) 116.00(12) P2 Ta2 N l 173.48(12) 90.33(8) P2 Ta2 N 5 73.76(12) 79.69(9) P2 Ta2 N 6 81.21(12) 74.77(9) Atom Atom Atom Atom Dihedral Angle (°) in 3.3 Dihedral Angle (°) in 3.4 T a l N l Ta2 N2 179.7(2) 166.99(17) PI T a l Ta2 P2 -115.32(5) 79.94(4) A l l N l T a l N 2 165.64(18) 165.90(14) Page S3 References begin on Page 96 Chapter 3 - Hydroalumination oj coordinated dinitrogen 3.6 The fate of the isobutyl group Excluding [NPN] amide migration from tantalum to aluminum, the most mechanistically important features of 3.3 and 3.4 that do not appear in the structure proposed for 3.1 are: 1) cleavage of the N - N bond, 2) the presence of two remaining hydrido ligands, and 3) the presence of only one isobutyl substituent on A l . To determine the origin of the two hydrides, isotopic substition experiments were performed. It is significant that the use of D2-l in this reaction does not change the chemical shift or the integration of either of the hydride resonances in the one-dimensional ' H N M R spectra of 3.3 or 3.4. The ' H / ' H C O S Y N M R spectra of these products still display the characteristic cross-peaks arising from scalar coupling between the hydrides. Thus, bridging hydrides are not the source of either of the hydrides in 3.3 and 3.4. G C - M S analysis of the headspace gas over a solution of D2-l and D I B A L in toluene showed D 2 exclusively and no H D . This extends the correlation between N - N bond cleavage and H 2 reductive elimination documented in Chapter 2. In light of the elimination of bridging hydrides in the formation of 3.3 and 3.4, the presence of two hydrides in 3.3 and 3.4 suggests elimination of isobutene, rather than any butane isomer. In addition to the 3 I P N M R spectra represented in Figure 3.2, alternating ' H N M R spectra were also recorded. These spectra allowed the observation of a singlet resonance at 6 4.63 ppm arising concurrently with 3.2, suggestive of the presence of an olefin. Two possible explanations for this event arise. The first is an event wherein T a l abstracts the p (methine) hydrogen from one isobutyl group via a six-membered ring as shown in Scheme 3.2. A similar process has been invoked in the DIBAL-media ted hydrogenation of a family of lanthanide amide complexes. 2 9 A b s t r a c t i o n b y T a - H A sh i f t s to A l -< Scheme 3.2 Page 84 Rejerences begin on Page 96 Chapter 3 - Hydroalumination of coordinated dinitrogen Given the spatial arrangement of the tantalum and aluminum atoms in 3.3 and 3.4, the alignment of the alkyl group in the intermediate at the center of Scheme 3.2 is plausible. This scheme implies the A l - H in 3.3 and 3.4 is the same H atom that arrived bound to the aluminum atom of D I B A L . The second possible explanation is the abstraction of the |3 proton by aluminum. The general (3 elimination reaction mediated by aluminum has been calculated to be more favorable for isobutyl substituents than with other alkyl groups. 1 8 It was pointed out in Section 3.1 that D I B A L can be obtained from triisobutylaluminum by p elimination. 2 3 Elimination mediated by aluminum is not ruled out by the experimental results. This possibility is shown in Scheme 3.3. Abstraction by Al - H A remains on T a [ N P N ] T a / ^ a f N P N ] [ N P N ] T a < ^ ^ J a [ N P N ] [ N P N J T a t f ^ ^ T a t N P N ] »( $>r — - "/ — • v>/ 3.1 Scheme 3.3 Since the difference between Scheme 3.2 and Scheme 3.3 is the position of the hydrogen atom originating on aluminum, diisobutylaluminum deuteride was employed to discriminate between these two possibilities. The absence of the upfield hydride resonance in the ' H { 3 I P } N M R spectrum, together with the observation of a singlet at 6 17.36 ppm, establishes that the latter event occurs preferentially. This also shows that the terminal and bridging hydrides in 3.1 do not exchange, as seen in the hydroborated derivatives of 1 presented in Chapter 2. 3.7 Nature of intermediate 3.2 Unstable intermediate 3.2 has not been fully characterized, but some comments on its structure are possible due to some findings from N M R spectroscopy. Complex 3.2 can be inferred to contain an intact N - N bond from the 1 : , N N M R spectrum of ,5N2-3.2 (acquired at low temperature), which shows J N N = 19.1 Hz . Al though ' H N M R spectroscopy of 3.2 in the absence of 3.1, 3.3, and 3.4 has proven impossible, resonances Page 85 Rejerences begin on Page 96 Chapter 3 - Hydroalumination ojcoordinated dinitrogen unique to 3.2 can be identified at 5 19.0 (terminal), 10.5, and 12.4 (bridging) ppm, suggesting hydrido ligands. Analysis of the methyl resonances in the ' H N M R spectrum indicates that 3.2 has only one isobutyl group. In consideration of all these observations, it seems intermediate complex 3.2 is likely as shown below. Me2 Ph Ph , S k / P h \ / h \ ^ \ \ Ph 3.2 The N - N bond is intact. The aluminum atom has lost one isobutyl ligand by p* elimination and retains the (3 hydrogen atom. | N P N | amide migration to aluminum and the reductive cleavage of the N - N bond satisfy the conversion of 3.2 to either 3.3 or 3.4, which are related by the rotation of the other I.NPNJ ligand. Based on the available information, it is not possible to choose definitively between elimination/bond cleavage preceding amide migration or an alternative explanation in which amide migration occurs first. However, a tantalum atom that had lost an amido ligand to aluminum should be less prone to the loss of additional ligands, making reductive elimination of bridging hydrides less facile. Conversely, amide migration from tantalum to aluminum is more likely if tantalum has other nitrogen ligands in its coordination sphere, as is probably the case after N - N bond cleavage and establishment of the bridging nitrido ligand of the T a 2 N 2 square. Therefore, amide migration should occur after elimination/cleavage in the conversion of 3.2 to the final products. 3.8 Interconversion of 3.3 and 3.4 Stable diastereomers 3.3 and 3.4 are consistently obtained in roughly equimolar amounts as end-products of the reaction given in Scheme 3.1, with the best combined Page 86 Rejerences begin on Page 96 Chapter 3 - Hydroalumination of coordinated dinitrogen yield of these two species being 88% (as measured by recovered mass - a similar yield is indicated by 3 I P N M R spectroscopy against internal standard). 3 I P N M R resonances of one isomer appear in Q D 6 solutions of the other i f they are left for over two weeks. To see i f these two complexes were interconvertible, 2:1 solutions of 3 . 3 and 3 . 4 were subjected to heating and 3 ' P N M R spectroscopy using an internal reference standard. It was found that heating restored a 1:1 ratio of 3 . 3 to 3 . 4 without decreasing the overall integration of 3 I P - N M R active products versus the internal standard, irrespective of which complex was originally present in the greater amount. This confirms that processes involving the rotation of one [NPN] ligand with respect to the T a 2 N 2 core are possible and reversible. The formation of a greater amount of 3 . 3 than 3 . 4 observed after the low-temperature N M R spectroscopic analysis of 3 .2 suggests that 3 . 3 is the kinetic product of the decomposition of 3 .2 . 3.9 Overall mechanism and kinetics The sequential 3 1 P N M R experiments summarized in Figure 3.2 and Chart 3.1 are useful in mapping out the reaction manifold shown in Scheme 3.1. The solid-state molecular structures of 3 .3 and 3 .4 help to establish the steps required for their formation from 3 . 1 . Further details regarding these steps were obtained from isotopic substitution experiments and N M R spectroscopy. The complete series of transformations described in this Chapter is summarized in Scheme 3.4. Page 87 References begin on Page 96 Chapter 3 - Hydroalumination oj coordinated dinitrogen Scheme 3.4 The data presented in Chart 3.1 have inherent limitations that preclude reliable kinetic analysis of this system. First, because the hydroalumination reaction is completed before the experiment has begun, no information on this important step is available other than that it is quantitative as measured by 3 I P N M R spectroscopy. Second, potential inhomogeneity in T[ relaxation times for the nuclei that give rise to the many 3 I P N M R resonances observed in this reaction manifold means that the correlation between integration and concentration in solution is inexact, even though the initial concentration of 1, and therefore of 3.1, is known. This is compounded by error and inconsistency in integration. That said, the conversion of 3.1 into 3.2 and thence to 3.3 and 3.4 can be modeled as a set of connected first-order processes 3 0 with rate constants as illustrated in Scheme 3.5. There are significant competitive side-reactions to formation of 3.3 and 3.4 from 3.2, but the minor side products were not isolated. The possibility of an inhibitory effect by the elimination products, H 2 or isobutylene, was not explored. Isotope effects in studies Page 88 Rejerences begin on Page 96 Chapter 3 - Hydroalumination of coordinated dinitrogen with D2-l and / 5 / V 2 - l were not detected. Some further discussion of a proper study appears in Chapter 5. 3.10 Reactions of 3.3 with C-C multiple bonds A l u m i n u m atoms have previously been shown to facilitate intramolecular rearrangements in transition metal hydride complexes, 3 1 and therefore it was hoped that 3.3 and 3.4 would participate in N - C bond forming reactions with other small molecules. It is worth noting that neither 3 .3 nor 3.4 react with the smaller hydrocarbons ethylene, propylene, or acetylene via insertion at the A l - H moiety. Rather, they appear to react at the terminal tantalum hydride. These reactions occur cleanly over several days as observed by 3 1 P N M R spectroscopy. No solid-state structures for the insertion products have yet been established. The absence of the terminal hydride resonance in the ' H N M R spectra and slight changes in the chemical shift values of resonances in the 3 1 P { ' H } N M R spectra are characteristic of these reactions. N o reactions were observed with cyclohexene or phenylacetylene. 3.11 Summary and conclusions Hydroalumination and hydroboration of 1 appear to be related in that the initial step forms a new tantalum hydride and a new N-heteroatom bond to the terminal nitrogen without disrupting the N - N bond. The fact that hydroalumination is complete during mixing of reagents is a marked difference between these two processes. The increased Lewis acidity of aluminum versus boron may be responsible for the observed increase in rate, as may the ease with which trimeric D I B A L forms monomers as compared to the organoboranes. Both reactions lead to reductive elimination of bridging hydrido ligands as H 2 , followed by N - N bond cleavage. Again , these events are faster in hydroalumination than in hydroboration, but they occur after a facile |3 elimination of isobutylene mediated by aluminum. The preservation of this cascade of reactions subsequent to the first addition reaction shows that the chemical modification of 1 can be extended beyond boron hydride reagents. The yields of isolable well-characterized products incorporating Page 89 Rejerences begin on Page 96 Chapter 3 - Hydroalumination of coordinated dinitrogen dinitrogen-derived nitrido ligands are also improved, and although an [NPN] amide migrated from tantalum to aluminum, no cleavage of bonds internal to the ancillary ligand was observed. [NPN] ligand rotation was observed, implying that [NPN] ligand phosphines may not necessarily always be transoid in derivatives of 1, despite their arrangement in 1. The observation of [NPN] ligand rotation in 3.3 and 3.4 strengthens the assignment of complexes 2 .3 and 2.6 from Chapter 2 as sister complexes, since they differed only in substituents at boron and the relative dispositions of intact [NPN] ligands. Page 90 Rejerences begin on Page 96 Chapter 3 - Hydroalumination ojcoordinated dinitrogen 3.12 Experimental Section 3.12.1. General Considerations Unless otherwise stated, general procedures were performed according to Section 2.12.1. 3.12.2. Reagents. Diisobutylaluminum hydride ( D I B A L , 1.0 molar solution in hexanes) was purchased from Ald r i ch and used without purification. Neat D I B A L was obtained by rotary evaporation of the commercial solution. Purity was ascertained by N M R analysis of the hydroa luminat ion of a known quantity of benzophenone in dry Q D 6 . Diisobutylaluminum deuteride was prepared by literature methods.2 3 Ethylene, propylene, (Praxair) and acetylene (Matheson) were used as obtained from commercial gas suppliers. Phenylacetylene was distilled prior to preparation of a toluene solution of known concentration, which was used in synthesis. 3.12.3. Synthesis, Characterization, and Reactivity of Complexes Synthesis and NMR characterization of [NPN]Ta(H)(u.-H )2(\i-Tt]l:r\2-NNAl(C4H9)2)Ta[NPN], 3 .1 . A solution of 40 mg 1 in roughly 1 m L d8-toluene was added to 4.5 mg (0.32 mmol, 1 equiv) neat D i B A L - H and the two reagents were mixed using a Pasteur pipet prior to transfer into a 9" Wilmad N M R tube fitted with a Kontes valve. The sample was frozen in liquid nitrogen and flame-sealed before being thawed in an ethanol-liquid nitrogen slurry and inserted into the probe of a Bruker A V A - 4 0 0 N M R Spectrometer that had been cooled to - 6 0 ° C . ' H N M R (400 M H z , C V D 8 , 213 K ) : 6 -0.61, -0.55, -0.38, 0.17, -0.05, 0.48, 0.80, 0.83 (s, 3 H each, 24H total), S i C / / 3 ; 0.95 (b, 2H) ( C H 3 ) 2 C H C / / 2 A 1 ; 0.31, 0.41 (s, 3 H each) ( C / / , ) 2 C H C H 2 A 1 ) ; 0.66, 1.22, 1.34, 1.46, 1.53, 1.71, 1.79, 1.84 ( A M X , 8 H total), PCH2; 1.52 (s, ' H ), A l - H ; 6.32, 6.50, 6.64, 6.70, 6.83, 6.90, 6.94, 7.20, 7.32, 7.34 (phenyl protons, various multiplicities, total 19 H - some resonances obscured by solvent), 7.66 ( A M X , 2H , J H H = 7.1 Hz, J P H = 1.7 Hz , PPh o-H) 111 ( A M X , 2 H , J H H = 7.2 Hz, J P H = 1.3 Hz , PPh o-H), 10.11 (dd, 1H, V H H = 11.2 Hz , VHP= 12.8 Hz), 10.75 (vq, 1H, 2Jm =11.2 Hz , VH 1, = 11.1 H Z 27H P= 15.8 Hz) , Ta / /Ta ; 19.20 (dd, 1H, 2Jm =11.1 Hz , Page 91 Rejerences begin on Page 96 Chapter 3 - Hydroalumination ojcoordinated dinitrogen 2 J H P =48 .2 H z ) , T a / / . 3 , P { ' H } N M R ( 1 6 1 . 9 M H z , C 7 D 8 , 2 1 3 K ) : 5 12.1 (d, J P P = 18.31 Hz) , 14.5 (d, J P P = 18.31 Hz) ; [NPN] ligand. The product was not isolable and therefore no elemental analysis was obtained. Synthesis of "N2-3.l. 80 mg of ,5N2-l were treated with 9 mg neat D I B A L in a manner analogous to that reported above for 1 1 . 3 1 P { l H } N M R (161.9 M H z , C V D 8 , 213 K ) : 6 12.1 (ddd, J P P =18.31 Hz , J P N = 26.0 and 6.5 Hz) , 14.5 (d, (d, J P P = 18.31 Hz) ; [NPN] ligand. 1 5 N { ' H } N M R (40.6 M H z , C 7 D 8 , 213 K ) : 6 -26.23 (dd, J P N = 26.0 Hz , J N N = 18.7 Hz), N b ; -32.88 (dd, J P N = 6.5 Hz , J N N = 18.7 Hz) , N t . Spectroscopic data for lsN2-3.2. The sample of 11 described above was allowed to stand at room temperature for 8 h before being reinserted into the spectrometer. New resonances present: ' H N M R : 10.50 (dd, 1 H , V H H = 11.1 H z , 2 7 H P = 15.0 Hz) , 12.4(vq, 1H, V H H = 11.2 Hz, V H H = 10.6 H z V H P = 13.6 Hz), Ta/ /Ta; 19.0 (d, I H , V H 1 , = 10.6 Hz), Ta / / . 3 , P { ! H } N M R (161.9 M H z , C 7 D 8 , 213 K ) : 6 -10.9 (d, J P N = 4.06 Hz) , 11.2 (s); [NPN] ligand. 1 5 N { ' H } N M R (40.6 M H z , C 7 D 8 , 213 K) : 6 -25.56 (dd, J P N = 4.1 Hz , J N N = 17.9 Hz), N b ; 31.40 (d, J N N = 17.9 Hz), N t . Synthesis of [NPN-Al(H)C4H9]Ta(H)(n-N)(n-N)Ta[NPN], 3.3 and 3.4. To a stirred 20 m L toluene solution of 1 (0.5862 g, 0.465 mmol) was added dropwise 0.47 m L of a commercially prepared 1.0 M hexanes solution of diisobutylaluminum hydride in a glove box. A color change from reddish-brown to yellowish-brown was immediate. The solution was stirred for 24 h and the solvents were removed under vacuum. The resulting dark yellow-brown solid was taken up in hexanes and X-ray quality crystals of two different morphologies were recovered on a frit (281.9 mg, 45.1%, of 3 .3 and 270.0 mg, 43.2%, of 3 .4, overall yield 88.3%). The crystals were separated manually for elemental, X-ray crystallographic, and N M R spectroscopic analyses. Characterization of a«ri-[NPN-All(H)C4H9]Ta(n-lN)(^N)Ta[NPN], 3 .3. ' H N M R (400 M H z , C 6 D 6 , 300 K ) : 6 -0.62, -0.27, -0.15, -0.12, 0.03, 0.15, 0.19, 0.59 (s, 3 H each, 24H total, S i C / / , ) , 1.06, 1.24 (d, 3 H each, 3 J H H = 7 H z , ( C / / 0 2 C H C H 2 A 1 ) , 1.05 (m, I H ( C H 3 ) 2 C / / C H 2 A 1 ) , 0.57, 0.68 ( A M X , ' H each, (CH 3 ) 2 CHC/7 2 A1) , 0.62, 0.65, 1.19, Page 92 References begin on Page 96 Chapter 3 - Hydroalumination oj coordinated dinitrogen 1.36, 1.42, 1.58, 1.76, 1.82 ( A M X , 8 H total, P C / / 2 ) , 1.23 (d, I H , J H H =11.3 Hz , A l - H ) , 6.38, 6.52, 6.60, 6.84, 6.90, 6.95, 7.08, 7.12, 7.26, 7.32, 7.41 (phenyl protons, various multiplicities, total 22 H), 8.09 ( A M X , 2 H , J H H = 6.95 H z , J P H = 1.70 Hz , PPh o-H) 8.21 ( A M X , 2 H , J H H = 6.95 Hz , J P H = 1.28 Hz , PPh o-H), 17.36 (dd, I H , V H H = 11.3 Hz, V H P = 16.6 Hz, Ta//) l 3 C{ 'H> N M R (100.6 M H z , Q , H 6 , 300 K ) : 6 -1.52, 0.56, 2.62, 3.36, 3.63, 4.33,5.12, 5.94 (s, S i C H 3 ) , 0.53, 1.02, 1.50, 2.60, 14.08, 15.44, 18.40 (d, P C H 2 ) , 24.61 (b, A1-CH 2 ) , 21.17, 26.73, 28.53 (Al-'Bu), 136.71, 134.91 (ipso P - Q H 5 ) , 130.56, 133.00 (ortho P - C 6 H 5 ) , 119.25, 121.52, 122.70, 123.74, 124.04, 126.30, 128.33, 128.54, 128.91, 129.56 (phenyl ring carbons). Note some proton and carbon resonances were eclipsed by solvent. 3 1 P { 1 H } N M R (161.9 M H z , C 6 D 6 . 300 K ) : 6 -22.9 (s, [NPN] ligand), 19.8 (s [NPN] ligand). 2 9 S i - D E P T N M R (79.5 M H z , C 6 D f > , 300 K ) : 6 7.52 (d, 2 J P S | = 5.9 Hz) , 8.77 (d, 2 J P S , = 15.1 Hz) , 9.24 (d, 2 J P S , = 14.2 Hz), 11.32 (d, 2 J P S l = 12.9 Hz) ; [NPN] ligand. Ana l . Calcd for C 5 2 H 7 3 N 6 P 2 S i 4 T a 2 A l : C , 47.42; H , 5.47; N , 6.25. Found: C , 47.47; H , 5.56; N , 6.29. Characterizat ion of sjrt-[NPN-Al(H)C4H9]Ta((ji-N)(n-N)Ta[NPN], 3.4. ' H N M R (400 M H z , C 6 D r > . 300 K ) : 5 -0.58(3H), -0.22 (3H), -0.10 (6H), 0.16 (3H), 0.24 (3H), 0.36 (3H), 0.47 (3H) (s, 24H total, S i C / / 3 ) , 1.33, 1.87 (d, 3 H each, 3 J H H = 7 Hz , ( C / 7 j ) 2 C H C H 2 A l ) , 1.91 (m, I H ( C H 3 ) 2 C / 7 C H 2 A 1 ) , 0.95, 1.16 ( A M X , ' H each, ( C H 3 ) 2 C H C / / 2 A 1 ) , 0.87, 1.21, 1.32, 1.41, 1.69, 1.83 ( A M X , 5 H total, PCH2), 1.54 (d, I H , J H H = 12.4 H z , A l - H ) , 6.49, 6.77, 6.82, 6.89, 7.22, 7.34, 7.40, 7.95 (phenyl protons, various multiplicities, total 19 H), 8.15 ( A M X , 2H, J H H = 6.90 Hz , J P H = 1.58 H z , PPh o-H) 8.26 ( A M X , 2 H , J H H = 7.15 H z , J P H = 1.21 Hz , PPh o-H), 15.96 (dd, I H , V,,H = 12.4 H z , V H P = 17.8 Hz , Ta/7) Note some proton resonances were eclipsed by solvent and silylmethyl resonances. 3 I P { ' H } N M R ( C 6 H 6 , 300 K ) : 8 -5.8 (s, [NPN] ligand), 12.4 (s [NPN] ligand). Ana l . Calcd for C 5 2 H 7 3 N ( 1 P 2 S i 4 T a 2 A l : C , 47.42; H , 5.47; N , 6.25. Found: C, 47.49; H , 5.49; N , 6.23. Synthesis of /J7V2-3.3. A solution of /5N2-l was treated in a manner similar to the preparation of 3.3 and 3.4. No changes to the ' H N M R spectrum were observed compared to that of 3.3. 3 1 P { ' H } N M R (161.9 M H z , C 6 D 6 , 300 K ) : 6 -22.9 (dd, 2 J N P = Page 93 Rejerences begin on Page 96 Chapter 3 - Hydroalumination oj coordinated dinitrogen 16.7 and 8.8 Hz), 19.8 (s), [NPN] ligand. I 5 N { ' H } N M R ( C 6 D 6 300 K , 40 M H z ) 5 57.9 (d, 2 J N P = 16.7 Hz) , 291.8 (d, 2 J N P = 8.8 Hz). Synthesis of '5N2-3A. A solution of / 5 / V 2 - l was treated in a manner similar to the preparation of 3.3 and 3.4. No changes to the ' H N M R spectrum were observed compared to that of 3 . 4 . 3 1 P { ' H } N M R (161.9 M H z , C 6 D 6 , 300 K ) : 5 -5.8 (d, 2 J N P = 10.1 Hz), 12.4 (s), [NPN] ligand. 1 5 N { ' H } N M R ( Q D 6 300 K , 40 M H z ) 6 54.8 (s), 313.9 (d, 2 J N P = 10.1 Hz) . 3 1 P N M R spectroscopic investigation of the reaction of 1 with DIBAL. A 9" Wilmad N M R tube was charged with 40.2 mg 1 in roughly 1 m L Q D 6 and a sealed glass capillary tube containing neat P ( O M e ) 3 as an internal reference. The tube was sealed with a 5 mm rubber septum and wrapped with ParaFilm laboratory f i lm and inserted into the probe of a Bruker A V A - 4 0 0 N M R spectrometer. The spectrometer was programmed to observe consecutive sets of ' H { 3 1 P} and 3 I P { ' H } spectra every 15 minutes for 25 hours. After initial spectrometer calibration was performed, the sample was ejected and 4.5 mg neat D i B A L - H (0.32 mmol, 1 equiv) in roughly 0.25 m L C 6 D 6 was added as a bolus through the septum using a 20-gauge hypodermic needle. The reagents were mixed by brief inversion of the tube before the sample was returned to the probe and automated acquisition was begun. Individual resonances were integrated with respect to the internal standard. Similar experiments using D2-\ and ^ A ^ - l were also conducted. Synthesis of D2-3A and its decomposition. A toluene solution of D2-l in a sealed flask was treated in a manner similar to the preparation of 3 .1 , giving D2-3A. The solution was frozen in liquid nitrogen and sealed under vacuum, after which it was allowed to stir overnight. The headspace gas was analyzed by G C / M S , showing D 2 gas and no H D gas. Interconversion of 3.3 and 3.4. In a glove box, 2:1 and 1:2 mixtures of 3 . 3 and 3 .4 were dissolved in C 7 D 8 and transferred to an 8" Wilmad N M R tube, which was capped with a plastic stopper and sealed with ParaFilm laboratory fi lm. Samples were observed initially and again after 24 Page 94 Rejerences begin on Page 96 Chapter 3 - Hydroalumination ojcoordinated dinitrogen h, after which no change in integration values was observed. In both cases heating overnight at 60° C led to 1:1 mixtures with minimal loss of 3 1P-active species. Insertion reactions of 3.3. In a representative experiment, an 8" Wilmad N M R tube fitted with a Kontes valve was charged with ~20 mg (0.014 mmol) 3 .3 in C 6 D 6 . The sample was degassed by three freeze-pump-thaw cycles and 1 or 2 equivs of ethylene, propylene, or acetylene was added over a Schlenk line under vacuum using a bulb of known volume to control stoichiometry. Alternatively, phenylacetylene or cyclohexene were added to C 6 D 6 solutions of 3 .3 in a glove box. The tube was flame-sealed under vacuum and observed daily over the course of a week via 3 1 P { ' H } N M R spectroscopy. Resonances due to formation of a new complex were observed in some cases as noted in Table 3.2. ' H { 3 I P } N M R data for the product of ethylene insertion into the Ta -H bond of 3.4 ( Q D 6 , 300K, 400 M H z ) : 6 -0.59, -0.31, -0.18, -0.07, 0.00, 0.14, 0.19, 0.61 (s, 3 H each, 24H total, S i C / / 3 ) , 1.14, 1.34 (d, 3 H each, 3 J H H = 6.4 H z , ( C / ^ ) 2 C H C H 2 A 1 ) , 1.09 (m, 1H ( C H 3 ) 2 C / / C H 2 A 1 ) , 0.67, 0.74 ( A M X , 1H each, ( C H 3 ) 2 C H C / / 2 A 1 ) , 0.68, 0.81, 1.29, 1.38, 1.41, 1.62, 1.78, 1.89 ( A M X , 8 H total, P C / / 2 ) , 1.11 (s, 1H, A l - H ) , 1.82 (multiplet, 2 H , T a C / / 2 C H 3 ) 2.18 (multiplet, 3 H , T a C H 2 C / / , ) , 6.42, 6.51, 6.70, 6.84, 6.89, 7.18, 7.36, 7.39, 7.41 (phenyl protons, total 20 H , some eclipsed by solvent), 8.09, 8.21 (d, PPh o-H). Table 3 . 2 . 3 1 P N M R spectral data on test reactions between 3.3 and hydrocarbons. Hydrocarbon: New resonances in 3 1 P{ 'H} N M R spectrum: Ethylene 5 (ppm)-23.1 (s), 16.8 (s) Propylene 5 (ppm) -19.8 (s), 13.6 (s) Acetylene 6 (ppm)-10.5 (s), 7.4 (s) Cyclohexene None (no reaction) Phenylacetylene None (no reaction) Page 95 Rejerences begin on Page 96 3.13 References: Chapter 3 - Hydroalumination ojcoordinated dinitrogen (1) Housecraft, C. E. ; Sharpe, A . G . Inorganic Chemistry; 1st ed.; Pearson Education Ltd.: Harlow, 2001. (2) Downs, A . J.; Pulham, C. R. Chem. Soc. Rev. 1994, 23, 175. (3) Ziegler, K . ; Gellert, H . - G . ; Kuhlhorn, H . ; Martin, K . ; Meyer, K . ; Nagel, K . ; Sauer, H . ; Zosel, K . Angew. Chem. 1952, 64. (4) Eisch, J. J . ; Hordis, C. K . J. Am. Chem. Soc. 1971, 93, 4496. (5) Eisch, J. J . ; Hordis, C. K . J. Am. Chem. Soc. 1971, 93, 2974. (6) Eisch, J. J. In Comprehensive Organometallic Chemistry; Wilkinson, G . , Stone, F. G . A . , Abe l , E. W. , Eds.; Pergamon Press Ltd.: Oxford, 1982; V o l . 1. (7) Turley, J. W. ; Rinn, H . W . Inorg. Chem. 1969, 8, 18. (8) Goncharenko, I. N . ; Glazkov, V . P.; Irodova, A . V . ; Somerikov, V . A . Physica B 1991, 774, 117. (9) Yoon, N . M . ; Brown, H . C. J. Am. Chem. Soc. 1968, 90, 2927. (10) Cotton, F. A . ; Wilkinson, G . ; Muri l lo , C. A . ; Bochmann, M : Advanced Inorganic Chemistry; 6th ed.; John Wiley & Sons, Inc.: New York, 1999. (11) Ayres, D. C ; Ki rk , D . N ; Sawdaye, R. J. Chem. Soc. B 1970, 1133. (12) Bottin, J . ; Eisenstein, O.; Minot, C ; Nguyen Trong, A . Tetr. Lett. 1972, 3015. (13) Greenwood, N . N . Chem. Soc. Rev. 1992, 21, 49. (14) Ecker, A . ; Weckert, E . ; Schnockel, H . Nature 1977, 387, 387. (15) Dohmeier, C ; Loos, D. ; Schnoeckel, H . Angew. Chem., Int. Ed. 1996, 35, 129. (16) Gauss, J.; Schneider, U . ; Ahlrichs, R.; Dohmeier, C ; Schnoeckel, H . J. Am. Chem. Soc. 1993, 775, 2402. (17) Ziegler, K . ; Gellert, H . - G . ; Lehmkuhl, H . ; Pfohl, W. ; Zosel, K . Liebigs Ann. Chem. 1960, 629, 1. (18) Bundens, J. W. ; Francl, M . M . Organometallics 1993, 72, 1608. (19) Seidel, W . Z. Anorg. Allg. Chem. 1985, 524, 101. (20) Grosse, A . V . ; Mavity, J. M . Journal of Organic Chemistry 1940, 5, 106. (21) Petrie, M . A . ; Power, P. P.; Wehmschulte, R. J. ; Rasika Dias, H . V . ; Ruhlandt-Senge, K . ; Waggoner, K . M . Organometallics 1993, 12, 1086. (22) Lalama, M . S.; Kampf, J. ; Dick, D. G . ; Oliver, J. P. Organometallics 1995,14, 495. (23) Egger, K . W . J. Am. Chem. Soc. 1969, 91, 2867. Page 96 References begin on Page 96 Chapter 3 - Hydroalumination ojcoordinated dinitrogen (24) Egger, K . W . Helv. Chim. Acta 1972, 55, 1502. (25) Pavia, D. L . ; Lampman, G . L . ; K r i z , G . S. Introduction to Spectroscopy; 2nd. ed.; Saunders College Publishing: Orlando, 1996. (26) Ebsworth, E. A . ; Rankin, D. W . H . ; Cradock, S. Structural Methods in Inorganic Chemistry; C R C Press: Boston, 1991. (27) Fryzuk, M . D. ; Clentsmith, G . K . B . ; Rettig, S. J. Inorg. Chim. Acta 1997, 259, 51. (28) Orpen, A . G . J. Chem. Soc, Dalton Trans. 1980, 2509. (29) Kl impel , M . G . ; Sirsch, P.; Scherer, W. ; Anwander, R. Angew. Chem., Int. Ed. 2003, 42, 574. (30) Connors, K . A . Chemical Kinetics: The Study of Reaction Rates in Solution; V C H : New York, 1992. (31) Bruno, J. W. ; Huffman, J. C ; Caulton, K . G . J. Am. Chem. Soc 1984, 106, 444. Page 97 References begin on Page 96 Chapter 4 Hydrosilylation of coordinated dinitrogen 4.1. Introduction The preceding two chapters introduce derivatives of 1 prepared by addition reactions of group 13 hydrides across the Ta -N t TC bond to give new tantalum hydrides and new N-heteroatom bonds. These complexes consistently exhibit spontaneous N - N bond cleavage correlated to reductive elimination of bridging hydrides as H 2 . Other rearrangements vary with the hydride reagent. Hydroborated derivatives show [ N P N | ligand degradation via s i ly l group migration to a new nitride. The change to an alkylaluminum hydride accelerates the addition reaction and the N - N bond cleavage event. Although no bonds internal to the | N P N | ligands are broken, amide migration from tantalum to aluminum is observed. The research described in this chapter concerns hydrosilylation of 1 and the reaction patterns of the hydrosilylated derivatives. Preliminary work carried out in this laboratory suggested that silanes should be a good choice for reaction with 1.' Like 1, the side-on bimetallic dinitrogen complex ( |P 2 N 2 JZr) 2 ([x-r | 2 :r | 2 -N 2 ), introduced in Section 1.6 (Equation 1.12 on Page 16), features a highly activated N 2 ligand that is accessible to electrophilic reagents. This complex reacts with /i-butylsilane via heterolytic H-S i bond activation to form a new N - S i bond and a new bridging hydride, as shown in Equation 4.1. It reacts with H 2 in a similar manner.2 [ P 2 N 2 ] [ P 2 N 2 ] Zr N (4.1) Zr [ P 2 N 2 ] Zr [ P 2 N 2 ] t\ 'age 98 References begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen The diverse reactivity of the S i - H bond has been compared with that of the H - H bond.3"6 The S i - H bond can form o complexes w i t h 7 8 and add oxidatively to transition metals. 9 1 0 A tautomeric equilibrium between a a complex and a silyl hydride complex has been observed, 3 as has Si H M electron-deficient bonding. 1 1" 1 4 These metal-silane interactions are illustrated in Scheme 4.1. L N M N + L N M ( N + 2 > + L N M N + H - L S J H 2 R H S l H 2 R H l v S J H 2 R oxidative electron-o complex addition deficient product bonding Scheme 4.1 Silanes have been employed as reducing agents for unsaturated substrates'3 and main group halides. 1 6 Hydrosilylat ion of olefins and other unsaturated substrates generally requires catalysis, although free-radical additions are known. 1 7 Olef in hydrosilylation is well developed and commercialized, primarily using the Speier and Karstedt platinum catalysts. 1 8" 2 0 Several mechanistic proposals for catalyzed hydrosilylation have been advanced. The Chalk-Harrod mechanism is a widely accepted view in which silane adds oxidatively to a preformed olefin-metal % complex . 2 1 2 4 Regiospecificity of this reaction is limited because olefin insertion into the M - H bond prior to elimination of product is reversible. A n alternate theory involving formation of the s i ly l hydride complex prior to interaction with the substrate has also been proposed. 3 , 2 3" 2 8 Early metals in high oxidation states catalyze hydrosilylation by a bond metathesis29"31 and can also generate polysilanes. 3 2 A recent finding suggests that a three-coordinate silylene may be an important intermediate in catalyzed hydrosilylation, and that the reaction may proceed via a concerted four-centered intermediate like that proposed in the hydroboration reaction. 3 3 A three-coordinate si lyl ium intermediate has also been suggested in Lewis acid-catalyzed hydrosilylation used in organic synthesis and surface functionalization. 3 4" 3 7. Five-coordinate 3 8" 4 2 and six-coordinate 4 3" 4 3 si l icon species are known to exist, allowing for hypervalent intermediates in hydrosilylation. Page 99 References begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen 4.2. Hydrosilylation of 1 with n-butylsilane The reaction between 1 and «-butylsilane proceeds cleanly and in high yield over four to twelve hours depending on reaction conditions. Solutions undergo a color change from dark brown to a distinct dark red. If the reaction is followed by 3 1 P N M R spectroscopy, sharp doublet resonances of a new material form as the resonances of 1 disappear. The reaction product has inequivalent phosphine ligands, with resonances at 6 8.9 and 23.3 ppm coupled by J P P = 17.8 Hz . These values, and the time scale of the reaction are reminiscent of hydroboration of 1. The presence of eight s i ly l methyl resonances in the ' H N M R spectrum at room temperature and at - 6 0 ° C indicates C , solution symmetry. A resonance suggestive of a new terminal tantalum hydride is observed at 5 14.3 ppm. Resonances indicating two bridging hydrides are present, but are inequivalent. Resonances assignable to a - S i H 2 " B u group are also present, indicating the n-butylsilyl fragment had been successfully incorporated into the new complex. This data suggests that addition of the H - S i bond across the T a - N bond has occurred, giving [NPN]Ta(H)((x-H) 2 ( (x-Ti ' :Ti 2 -NNSiH 2 "Bu)Ta[NPN], 4.1, as shown in Equation 4.2. Ph (4.2) n Bu This assignment was validated by the solid-state molecular structure, which was determined by X-ray crystallography. A n O R T E P drawing of 4.1 is shown in Figure 4.1. Selected bond distances and angles are listed in Table 4.1. Page 100 References begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Figure 4.1. O R T E P drawing (ellipsoids at 50% probability) of the molecular structure of |NPN]Ta(H ) (^ i -H) 2 (n - r i , :Ti 2 -NNSiH 2 n Bu)Ta[NPN] , 4.1. S i ly l methyls and phenyl ring carbons other than ipso omitted for clarity. Hydrides were modeled using X - H Y D E X . 4 ' ' Table 4.1. Selected bond distances (A), bond angles (°), and dihedral angles (°) for |NPN]Ta(H) (u -H) 2 (u -T i ' : r f -NNSiH 2 "Bu)Ta |NPN] , 4.1. Atom Atom Distance (A) Atom Atom Distance (A) N I N 2 1.363(13) T a l NI 2.181(10) T a l N2 2.179(10) Ta2 N2 1.835(10) NI S i l 1.732(11) T a l PI 2.583(3) T a l N3 2.028(10) T a l N4 2.136(11) Ta2 P2 2.632(3) Ta2 N 5 2.044(8) Ta2 N6 2.075(7) T a l Ta2 2.8430(9) Page 101 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Atom Atom Atom Angle O Atom Atom Atom Angle O T a l N I S i l 133.6(5) T a l N I N 2 71.7(6) T a l N 2 Ta2 89.7(4) N I T a l PI 79.3(3) N I T a l N3 159.5(4) NI T a l N 4 87.0(4) N 2 Ta2 P2 166.6(3) N2 Ta2 N 5 108.4(4) N2 Ta2 N6 105.8(4) Atom Atom Atom Atom Dihedral Angle (°) N I T a l Ta2 N 2 1.0(5) Ta2 T a l NI S i l 105.7(8) PI T a l Ta2 P2 -170.6(2) The spatial arrangement of the [NPN] ligand on T a l is quite similar to that observed in hydroborated complex 2.1, in that one face of the metal is left vacant. Again, this face is likely occupied by the terminal hydride, which can be modeled in this position using X - H Y D E X . 4 6 In fact, most of the metric parameters of these two complexes are nearly identical, with two exceptions that pertain to the dinitrogen fragment. First, the N -N bond length of 1.363(13) A is not significantly elongated in 4.1 as compared to 1.319(4) A in 1. Hydroborated complex 2.1 showed an N - N bond length of 1.411(15) A. Second, a notable parameter not included in Table 4.1 is the sum of bond angles around N I , which is 319.8° in 4.1. The sum of angles around N I in 2.1 is 354.6°, which indicates nearly planar geometry at N I . In 2.1 the potential for N - B TC interactions exists because of a vacant p orbital on boron. The distribution of the alkyl groups around B l , the hybridization of B l , and the coplanarity of B l with the T a 2 N 2 core all support this scenario. S i l lacks a similar vacant orbital, and therefore there is no potential for this type of interaction between N I and S i l in 4.1. Furthermore, this suggests that N - B TC interactions are withdrawing electron density from N - N bonding orbitals in 2.1, which may account for the increased N - N bond elongation as compared to 1. The probable absence of these interactions in 4.1 allows the n-butylsilyl group to reside in a position that is distinctly cis to the site of the terminal hydride. Two resonances were observed in the l 5 N { ' H } N M R spectrum of ,5N2-4.1. The resonance at 5 -52.5 ppm is a doublet of doublets, showing coupling to the other resonance by ' J N N = 16.6 Hz , and a second coupling that is also observed in the downfield Page 102 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen 3 I P N M R resonance (% N = 26.4 Hz) . This resonance can be assigned to N b , or N2 in Figure 4.1. The other resonance at 8 -163.5 ppm displays only the N - N coupling. This resonance can be assigned to N„ or N l in Figure 4.1. The N - N scalar coupling is expected based on the molecular structure of 4.1, which indicates an intact N - N bond not unlike that in 1. These resonances are quite like those of complex / J7V2-2.1, and the similar drastic change in the chemical shift for N , versus parent complex J5N2-1 is another characteristic sign of functionalization at the terminal N atom. A ' J N S i = 5.2 Hz coupling is evident in the «-butylsilyl resonance in the 2 9 S i - D E P T N M R spectrum of /5A/2-4.1. This finding shows the E - H addition across the T a - N bond of 1 can be extended beyond group 13 hydrides to other hydride reagents. 4.3. Facile N-N bond cleavage Solutions of 4.1 are thermally sensitive and convert completely over four to six hours at room temperature. If this reaction is monitored by 3 I P { ' H } N M R spectroscopy, two resonances characteristic of a new species, complex 4.2, can be seen emerging as the resonances of 4.1 disappear. These broad resonances ( F W H M = 28 Hz) are both upfield from the chemical shift range typical of the [ N P N | T a complexes, at 8 -5.7 and -12.3 ppm. Their line shapes are very similar, and no coupling information can be extracted from them even at lower temperatures. The ' H N M R spectrum at room temperature implies C s symmetry, and this is supported by the 2 9 S i - D E P T N M R spectrum. This spectrum has three features that can be assigned as one butylsilyl and two ligand silyl resonances on the basis of 3 ' P couplings and ' H / 2 9 S i correlational spectroscopy. Although a terminal hydride resonance remains in the ' H N M R spectrum, 4.2 lacks resonances indicative of bridging hydride ligands. The status of the dinitrogen fragment was explored using l : , N { ' H } spectroscopy. In hN2A.2, a resonance suggestive of a bridging silylimide at 8 -44.8 ppm is present in the ' ^ { ' H } N M R spectrum, with an 18.7 H z coupling matching the coupling observed in the broad 3 1 P N M R resonance at 8 —12.3 ppm. The other resonance is a singlet, far downfield at 8 284.4 ppm. This is a reasonable chemical shift for a bridging nitrido l i g a n d 4 7 , 4 8 In the initial report of this chemistry 4 9 the probable structure of 4.2 was assigned as [NPNJTa(H)([x-N)(u J-NSiH 2 nBu)Ta[NPNJ based Page 103 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation ojcoordinated dinitrogen on consideration of the chemical shift of this nucleus and the absence of N - N scalar coupling between these resonances. Equation 4.3 shows the reaction. (4.3) A fluxional process involving a rotation of the leftmost facially bound [NPN] ligand in 4 .2 about the Ta-P bond such that the ligand amides are rendered equivalent would account for the observed solution symmetry. The hydride is also required to move in this scenario, but this should be facile due to the comparatively small size of the hydrido ligand. Shortly after publication of the initial report, the solid-state molecular structure of 4 .2 was established by X-ray crystallography. Figure 4.2 shows an O R T E P drawing of this structure, which confirms the proposed structural assignment of 4.2. Table 4.2 summarizes key bond lengths and angles in 4.2. Page 104 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen F i g u r e 4 . 2 . O R T E P drawing (ellipsoids at 50% probability) of |NPN]Ta(H)(u^N)(u-NSiH 2 n Bu)Ta[NPN| , 4 .2 . S i ly l methyls and phenyl ring carbons other than ipso omitted for clarity. The hydride was modeled using X - H Y D E X . 4 6 Table 4 .2 . Selected bond distances (A), bond angles (°), and dihedral angles (°) for | N P N |Ta(H)(u-N)(uNSiH 2 "Bu)Ta | N P N |, 4 .2 Atom Atom Distance (A) Atom Atom Distance (A) NI N 2 2.646(6) T a l Ta2 2.9258(6) T a l N 2 1.928(11) Ta2 N 2 1.874(9) T a l NI 2.187(10) Ta2 NI 1.924(10) NI S i l 1.741(10) T a l PI 2.709(3) Ta l N3 2.107(9) T a l N 4 2.034(9) Ta2 P2 2.715(3) Ta2 N 5 2.079(10) Ta2 N6 2.052(9) Page 105 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation ojcoordinated dinitrogen Atom Atom Atom Angle (°) A tom Atom Atom Angle O T a l N l S i l 127.7(5) Ta2 N l S i l 139.2(5) T a l N l T a l 90.5(4) T a l N 2 Ta2 100.6(5) N l T a l N 2 80.0(4) N l Ta2 N 2 88.6(4) N2 Ta2 P2 94.1(3) N2 T a l PI 170.7(3) N3 T a l N 4 100.3(4) N 5 Ta2 N 6 118.0(4) Atom Atom Atom Atom Dihedral Angle (°) N l T a l Ta2 N2 174.0(6) PI T a l Ta2 P2 -163.66(13) Each of the phosphines is trans to a nitrogen atom. It has been proposed that the broadness of one of the 3 I P N M R resonances of 1 is due to its trans disposition to quadrupolar nitrogen atoms. 3 0 If so, then the broadness of both resonances of 4.2 is easily explained. Since the broadness of these peaks persists in /5N2-4.2, in which the quadrupolar nuclei in the dinitrogen fragment are replaced with non-quadrupolar l 3 N nuclei (I = 1/2), it seems that anomalous peak widths in the 3 ' P N M R spectra of the [ N P N | T a complexes may be due to other factors in addition to quadrupolar broadening effects. Complex 4.2 is significant to this entire thesis because it firmly establishes the proposed role of reductive elimination of bridging hydrides as H 2 in the cleavage of the N - N bond. Arguments based solely on the absence of scalar J N N coupling in 1 5N 2-4.2 were persuasive but not conclusive. The solid-state structure of 4.2 unequivocally shows that N - N bond cleavage occurs. The T a - N bond distances are longer for silylimido N l than for the bridging nitride N 2 . The shortest T a - N bond is between Ta2 and N2, which supports the placement of the Ta=N double bond in the simple drawing of 4.2 shown in Equation 4.3. Minor asymmetries in bond lengths and interior angles over such M 2 N 2 motifs are common for the group 5 metals. 5 1 , 3 2 4.4. Monitoring hydrosilylation of 1 by31P NMR spectroscopy In Chapter 3, a series of consecutive timed 3 1 P N M R experiments using an internal standard for integration was a key experiment in understanding the reaction pathway and some important intermediates in the hydroalumination reaction. This same Page 106 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen approach was applied to the hydrosilylation reaction in an effort to shed light on this transformation. The hydrosi lylat ion reaction giving 4 . 1 is not as rapid as hydroalumination, and therefore four equivalents of n-butylsilane were used to force the reaction kinetics into an accessible time frame, given the practical limits on spectrometer access. The first three hours of this reaction path are shown in the stack plot in Figure 4.4. Resonances of 1 (brown) are almost completely converted to those of 4.1 (sharp doublets at 5 23.3 and 8.9 ppm, green) over this time period. The broad upfield resonances characteristic of 4 .2 (blue) are observed to grow in as the peaks due to 4.1 begin to disappear. 2 5 2 0 1 5 i • 10 0 - 5 - 1 0 p p m Figure 4 .3 . 3 ' P N M R spectra acquired at fifteen-minute intervals over the first three hours of the reaction between 1 and four equivalents n-butylsilane. One surprising feature is the appearance of a small singlet very near 0.0 ppm, marked with an asterisk in Figure 4.3. Figure 4.4 shows that this resonance (complex 4 .3 , red) continues to grow in over the course of the next few hours. This figure shows spectra acquired every 75 minutes over the entire sixteen-hour experiment. The spectral width Page 107 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen has been adjusted to show that another species, 4.4 (5 24.2 and 71.2 ppm, yellow), appears after five hours. 70 60 50 40 30 20 10 0 -10 ppm Figure 4.4. " P N M R spectra for the entire sixteen-hour experiment Neither of these species is observed in 1:1 reactions, which implies that other reactions between the silylated derivatives of 1 and the silylating reagent may be possible. When 2:1 reactions between A2-butylsilane and 1 are performed, a dark red solid with the characteristic " P N M R spectrum of complex 4.3 is isolated in moderate yield. The solution symmetry of this species is of interest. Most derivatives of 1 feature two inequivalent phosphines, and the chemical shifts and coupling constants of the resonances can be characteristic of molecular structure. Observation of a singlet in the 3 1 P N M R spectrum of 4.3 suggests a monomeric complex or a dimeric (or higher order) complex of appropriate symmetry. The ' H N M R spectrum of 4.3 is ambiguous in that only two [NPN] ligand silyl methyl environments and one A M X doublet of doublets for the ligand methylene resonances are observed. Resonances characteristic of an ^ - b u t y l s i l y l Page 108 References he gin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen resonance are also present, but no resonances suggestive of hydride ligands are observed. Only two resonances are present in the 2 9 S i { ' H } spectrum, and the ' H / 2 9 S i H S Q C experiment depicted in Figure 4.5 clearly shows that one of these correlates to [NPN] ligand silylmethyl and methylene resonances, while the other correlates to the /r-butylsilyl Si/7 2 and S i C / / 2 resonances. 9 8 7 6 5 4 3 2 1 0 ppm Figure 4.5. ' H / 2 9 S i H S Q C correlational spectrum of 4.3. The solid-state molecular structure of 4.3 was originally determined from a twinned crystal, and this structural information was included in the initial report of this chemistry. 4 9 Since that report, a single crystal has been prepared and analyzed by X-ray diffraction. The results of these two studies are the same. The molecular structure of 4.3 is shown in Figure 4.6. Selected bond lengths and angles are presented in Table 4.3. Page 109 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Figure 4.6. O R T E P drawing (ellipsoids at 50% probability) of the solid-state molecular structure of ( | N P N | T a ) 2 ( L i - N S i H 2 n B u ) 2 , 4 .3 . S i ly l methyls and phenyl ring carbons other than ipso omitted for clarity. Table 4 .3 . Selected bond distances (A), bond angles (°), and dihedral angles (°) for ( |NPN|Ta) 2 (u -NSiH 2 "Bu) 2 , 4.3. Atom Ato m Distance (A) Atom Atom Distance (A) N I N 2 2.935(4) Ta l N I 1.997 (9) T a l N 2 2.007(9) Ta2 NI 2.038(10) Ta2 N2 1.990(9) NI S i l 1.729(10) N 2 Si2 1.739(10) T a l Ta2 2.6677(6) T a l PI 2.735(3) T a l N3 2.070(9) T a l N 4 2.045(9) Ta2 P2 2.687(3) Ta2 N 5 2.077(9) Ta2 N6 2.069(8) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) T a l N I Ta2 82.8(4) NI Ta2 N 2 93.5(4) Ta2 N2 T a l 83.7(3) N2 T a l NI 94.3(4) T a l NI S i l 143.2(6) Ta2 N2 Si2 134.8(5) Atom Atom Atom Atom Dihedral Angle (°) Ta l N2 Ta2 NI 155.8(5) PI T a l Ta2 P2 145.48(13) Page 110 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation ojcoordinated dinitrogen Two /7-butylsilyl groups have been incorporated into 4 .3 as substituents on the dinitrogen-derived N atoms. These units can be formulated as bridging silylimides. The idealized symmetry of this molecule is C 2 h , which explains the solution N M R spectra. The solid-state molecular symmetry is lower due to bending of the T a 2 N 2 core and canting of the n-butylsilyl fragments in the same direction. The dinitrogen-derived atoms have both been silylated in a chemically identical manner even though they are asymmetrically activated in 1 and in 4 .1 . The reaction between 4 . 2 and a second equivalent of n-butylsilane can be presumed to proceed as shown in Equation 4.4, which includes the bimetallic reductive elimination of H 2 to give a new Ta-Ta bond to account for the observed diamagnetism. This is a viable method of directly synthesizing 4 .3 in yields similar to the 2:1 reaction between «-butylsilane and 1. A similar instance of the silylation of a tantalum imido ligand is known, although reductive elimination of H 2 did not feature in that report. 5 3 The metal-metal bond in 4 .3 may offer reactivity characteristic of metal-metal bonded species."14 Reactions that form new bonds to the imido ligands are of particular interest. Test reactions between C O or C 0 2 and 4 . 3 produce a variety of insoluble products that resist characterization. No reaction is observed between 4 .3 and olefins, and it does not react with H 2 even under 30 atmospheres. The stability of early metal imides may contribute to the observed lack of reactivity."0 The synthesis of complex 4 . 3 from 1 represents a scheme that cleaves N 2 and functionalizes both N atoms in an equivalent manner, despite the asymmetric manner of N 2 activation exhibited by 1. The remainder of this Chapter explores schemes aimed at synthesizing homologs of 4.3 and their structures and properties. Page III Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen 4.5. Rearrangements due to excess silane Figure 4.4 indicates the existence of complex 4 .4 , which has 3 I P N M R resonances at 8 24.2 and 71.2 ppm. These are observed to arise together after the appearance of 4 .3 has begun, and this suggests that some species in the reaction manifold is undergoing a further hydrosilylation. Solutions of pure 4 .3 are stable for weeks, but the 3 I P N M R resonances of 4 .4 can be seen to develop if additional silane is present. The solid-state molecular structure of 4 .4 has been determined by X-ray crystallography, and an O R T E P drawing of this structure is shown in Figure 4.7. Selected bond lengths and angles are listed in Table 4.4. F igu re 4 . 7 . O R T E P drawing (ellipsoids at 50% probability) of the molecular structure of |NP(Ph)N |Ta( n BuSiH 2 -N-S i (H) n Bu -L i -N)Ta |NPN] 4.4 Silyl methyls and phenyl ring carbons other than ipso omitted for clarity. Page 112 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen T a b l e 4 .4 . Selected bond distances (A), bond angles (°), and dihedral angles (°) for [NP(Pn)N]Ta( n BuSiH 2 -N-Si (H) n Bu-^-N)fa[NPN] , 4.4. Atom Atom Distance (A) Atom Atom Distance (A) N l N 2 2.549(6) T a l Ta2 2.9511(5) T a l N l 2.103(8) T a l N 2 2.073(7) Ta2 N 2 1.972(8) N l S i l 1.696(8) N l Si2 1.703(9) N2 Si2 1.742(8) T a l N3 2.065(7) T a l N 4 2.078(8) T a l PI 2.597(2) Ta2 N 5 2.085(8) Ta2 N6 2.095(7) Ta2 P2 2.637(2) Ta2 CIO 2.276(9) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) S i l N l Si2 128.4(5) N l T a l N 2 75.2(3) N l Si2 N 2 95.4(4) T a l N 2 Ta2 93.7(3) Ta2 T a l PI 83.59(5) T a l Ta2 P2 129.09(5) T a l Ta2 CIO 92.3(2) Atom Atom Atom Atom Dihedral Angle (°) N l T a l Ta2 N2 4.1(4) N l T a l Ta2 P2 9.2(3) PI T a l Ta2 CIO 6.3(2) PI T a l Ta2 P2 -171.06(9) S i l N l Si2 N 2 178.8(7) (4.5) Complex 4 .4 , formulated as i ;NP(Ph)NJTaC 1 BuSiH 2 N-Si(H)"Bu-u J -N)Ta|NPN;|, is formed by a rearrangement of the symmetric T a 2 N 2 core of 4 .3 , as illustrated in Equation 4.5. The idealized C s symmetry implied by the simple drawing in Equation 4.5 is reflected in the solution ' H N M R spectra. In 4 .4 , both dinitrogen-derived N atoms are still silylated, but they are no longer on opposite sides of the Ta-Ta internuclear axis. N l has Page 113 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen been displaced from its bridging position in 4 . 3 , and it has formed another N - S i bond to the silyl atom on N2. The phenyl group of one [NPN] ligand phosphine has undergone C -H activation at its ortho position and is now part of a planar five-membered metallacycle. Four coupled aryl resonances indicative of the four unique protons on the cyclometalated phenyl ring are clearly evident in the ' H / ' H C O S Y N M R spectrum of 4.4. The observed diamagnetism of complex 4 .4 can be rationalized by another loss of H 2 via reductive elimination, with one H atom contributed by the phenyl ring and the other H atom contributed by Si2. This is supported by the ' H N M R spectrum of 4 .4, which has separate resonances suggestive of two equivalent H atoms on S i l but only one on Si2 (numbered as in Figure 4.7). Formal charge considerations suggest a Ta-Ta bond as shown in Equation 4.5, and the anomalously long T a l - T a 2 internuclear distance of 2.9511(5) A may be due to rigidity of the internal angles in the five-membered metallacycle. The requirement of additional silane for formation of 4 . 4 suggests that the mechanism by which 4 .4 arises is complicated. The observation of reversibility in the hydrosilylation of a tantalum imido ligand, 3 3 indicates that silylation of 4 .3 followed by rearrangement and loss of silane may be involved in this transformation. Solutions of 1 in neat /z-butylsilane form 4 .4 as the major product by 3 ' P N M R spectroscopy, implying that complex 4.4 is the endpoint of this reaction manifold, as shown in Scheme 4.2, and that further silylation of derivatives of 1 is not possible. [ N P N ] T a ^ ^ T a [ N P N ] " B u S i H , [ N P N ] T a ^ ^ T a [ N P N ] H N y H 2 S i \ n B u - H , 4 . 1 B u n x [NPN]Ta<^ ^ T a [ N P N ] H N / H 2 S i \ n B u [NPN]Ta I i / S i M e 2 n-rJ X-'Nl/,: SiHo excess " B u S i H 3 - T a - ^ . ' N ' P h 4 . 4 \ / S i H 2 n B u S iH I n B u n B u S i H 3 [NPN]Ta^- 7Ta[NPN] H 5 N H 3 HpSi 4.3 n B u 4 . 2 Scheme 4.2 Page 114 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen 4.6. Hydrosilylation of 1 with phenylsilane In hydroboration of 1, changing substituents results in minimal attenuation of the reaction outcomes. Would the same be true in hydrosilylation of 1? Complex 4.5 is available in quantitative yield and analytical purity from the reaction shown in Equation 4.6, indicating that the hydrosilylation step is not affected by changing substituents from n-butyl to phenyl. Ph (4.6) Ph The spectral characteristics of 4.5 are unremarkable in comparison to those of 4.1, except that the chemical shifts of the bridging hydride ligands are not separated to the same extent and appear as a virtual triplet in the ' H { 3 ' P } N M R spectrum at 400 M H z . This phenyl homolog of 4.1 crystallizes readily from hexanes and the solid-state molecular structure as determined by X-ray crystallography is shown in Figure 4.8. Selected bond lengths and angles are listed in Table 4.5. There are no significant differences between the metric parameters of 4.5 and 4.1. Page 115 References begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Figure 4.8. O R T E P drawing (ellipsoids at 50% probability) of the molecular structure of |NPN|Ta(H) (u -H ) 2 (^-r i 1 :T i 2 -NNSiH 2 Ph)Ta[NPN] , 4.5. S i ly l methyls and | N P N | ligand phenyl ring carbons other than ipso omitted for clarity. Hydrides were modeled using X - H Y D E X . 4 6 Page 116 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen Table 4.5. Selected bond distances (A), bond angles (°), and dihedral angles (°) for [ N P N I T a C H X f x - ^ ^ j x - r i ' i T i ' - N N S i H ^ ^ T a l N P N K ^ S . Atom Atom Distance (A) Atom Atom Distance (A) N I N 2 1.354(10) T a l N I 2.179(7) T a l Ta2 2.8393(14) T a l N 2 2.214(6) Ta2 N 2 1.860(8) NI S i l 1.736(7) T a l N3 2.137(7) T a l N 4 2.080(7) T a l PI 2.590(2) Ta2 N 5 2.049(7) Ta2 N6 2.041(8) Ta2 P2 2.635(3) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) T a l NI S i l 135.2(4) T a l N I N2 73.5(4) T a l N2 Ta2 87.9(3) N I T a l P I 80.96(18) NI T a l N3 161.7(3) N I T a l N 4 84.9(3) N 2 Ta2 P2 167.6(2) N 2 Ta2 N 5 103.6(3) N 2 Ta2 N6 108.7(5) Atom Atom Atom Atom Dihedral Angle (°) N I T a l Ta2 N 2 -0.7(3) Ta2 T a l NI S i l 106.9(5) PI T a l Ta2 P2 -169.70(14) Complex 4.5 shares the thermal instabil i ty observed for al l s imi lar ly functionalized derivatives of 1. After a few hours at room temperature, the red color of 4.5 in solution dissipates. Resonances of 4.5 convert to those of complex 4.6 when this process is observed by 3 I P N M R spectroscopy. Although the solid-state molecular structure of 4.6 has not yet been determined, the multinuclear N M R spectra of this species and its l ; ,N 2-labeled isotopomer do not suggest that it differs structurally from 4.2, and therefore the reaction can be considered to proceed as shown in Equation 4.7. Page 117 References begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen (4.7) If 4 . 6 and 4 . 2 are truly congeners, then 4 .6 should be a substrate for second hydrosilylations across the tantalum-nitride % bond. In 2:1 reactions between 1 and phenylsilane, no 3 I P resonance suggestive of a C 2 h -symmetric Z>is-phenylsilyl analog of complex 4 .3 is observed, but an ochre powder is recovered in 56% yield. This material, complex 4 .7 , has a very complicated ' H N M R spectrum indicative of C, solution symmetry. The 3 I P N M R spectrum features two singlets at 8 -10.5 and 18.9 ppm, which is not suggestive of any of the species observed in the hydrosilylation chemistry. Some complexes showing [NPNJ ligand activation in Chapter 2 showed roughly similar chemical shifts, although scalar couplings between 3 I P nuclei were generally observed in those complexes and none is resolvable in the 3 ' P spectrum of 4 .7 . Fortunately, single crystals of 4.7 suitable for X-ray diffraction were obtained. A n O R T E P depiction of the solid-state molecular structure of 4 .7 is shown in Figure 4.9, and selected bond lengths and angles for this structure are listed in Table 4.5. Page 118 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Figure 4.9. O R T E P drawing (ellipsoids at 50% probability) of 4.7, I N P N ]Ta (L i -N-S iH 2 Ph) (n -N-S iH 2 Ph)Ta[NPN |. S i ly l methyls and | N P N | ligand phenyl ring carbons other than ipso omitted for clarity. Table 4.6. Selected bond distances (A), bond angles (°), and dihedral angles (°) for |NPN|Ta(^i-N-S^iH 2 Ph)(u-N-SiH 2 Ph)Ta|NPNJ ,4 .7. Atom Atom Distance (A) Atom Atom Distance (A) NI N2 2.827(3) Ta l Ta2 2.9837(3) Ta2 N 2 1.903(5) Ta2 NI 1.911(5) T a l N 2 2.253(5) T a l NI 2.158(4) N 2 Si2 1.724(5) Si2 N 5 1.758(5) N I S i l 1.729(5) Si2 T a l 2.527(3) Ta2 N3 2.050(5) Ta2 N 4 2.035(5) Ta2 P2 2.7962(17) T a l N6 2.032(5) T a l PI 2.5034(17) Page 119 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Atom Atom Atom Angle O Atom Atom Atom Angle (°) T a l N 2 Ta2 91.4(2) T a l N I Ta2 94.1(2) NI Ta2 N 2 95.17(19) N I T a l N 2 79.30(18) N2 Si2 N 5 118.9(2) Ta2 T a l N 6 127.08(14) N3 Ta2 N 4 114.61(19) N 2 Ta2 P2 88.10(14) N6 T a l PI 82.47(15) Ta2 N I S i l 146.1(3) Atom Atom Atom Atom Dihedral Angle (°) N I T a l N2 Ta2 177.9 PI T a l Ta2 P2 73.31(10) Ta2 N 2 Si2 N 5 -108.9(2) N 2 Ta2 NI S i l 155.2(5) A n [NPN] ligand amide has migrated to Si2. In retrospect, the 3 I P N M R spectrum of 4.7 (chemical shifts and the absence of scalar coupling) is reminiscent of complexes 3.3 and 3.4 of the hydroalumination study in Chapter 3, which featured [NPN] ligand amide migration. Al though no s i ly l hydride ligands were located in the X- ray crystallographic analysis of 4 .7 , each phenylsilyl group shows couplings to two different S i - H protons. One of these S i - H resonances is at 5 9.43 ppm, which is anomalously downfield for a silyl hydrogen. This may be suggestive of an agostic interaction between one H atom on Si2 and T a l . V o 7 The S i - H coupling constant for this proton is 78.2 Hz, whereas the other ' J S i H couplings in the complex range from 114 to 168 Hz. This suggests that the hydrogen atom is still bound primarily to Si2 but is interacting with T a l . A small coupling to 3 , P is also evident in this resonance. A drawing of indicating the presence of electron-deficient Ta H-Si bonding involving one H atom in 4.7 is shown below. P h F P h / P h M e 2 S i ^ S i H \ / ^ . S i - ^ N ^ N / \ S i M e 2 / N N ' p h 1 I » I P h H 2 S i \ 4.7 P h P h Page 120 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen There is no suggestion of a similar [NPN] ligand amide migration in the chemistry of 1 with /i-butylsilane. In fact, the conversion of 4 .3 to 4 .4 suggests that when /z-butyl groups are present on the si lyl addition reagent, the [NPN] ligand amides can remain bound to the tantalum atoms through extreme rearrangements of other portions of the molecule. The contrast between this finding and the amide migration observed in 4.7 indicates that s i ly l substituents play a role in organizing these complexes. T w o mechanistic possibilities arise. In the first scenario, 4 .7 results directly from amide migration in an intramolecular rearrangement of a complex that is isostructural with 4 .3. In the second scenario, the second equivalent of phenylsilane interacts with a ligand amide of 4 .6 in an initial step and then with the bridging nitride to give 4 .7 . This path implies poorer regioselectivity in the second hydrosilylation with phenylsilane versus n-butylsilane. These two alternatives are expressed in Scheme 4.3. hydrosilylation at amide \ H 2 S i Ph hydrosilylation P h S i H 3 at nitride ,, w - H 2 Ph -H 2 nitride attack Ph r Ph amide migration Me 2 Si SiH Ph Ph H 2 S ' y Ph H 2 S i \ Ph 4.7 Scheme 4 .3 Page 121 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen A simple experiment was undertaken to explore these possibilities. It has already been shown that symmetrically functionalized complex 4.3 is available by direct hydrosilylation of the dinitrogen-derived bridging nitrido ligand of 4.2. This suggested that complexes isostructural to 4.3 but incorporating two different substituents should be obtainable. The synthesis of complex 4.8 was undertaken as shown in Equation 4.8. (4.8) The 3 I P N M R spectrum of 4.8 comprises a doublet of doublets ( J P P = 9.9 Hz), with both peaks very near the location of the single resonance observed for 4.3. The ' H and 2 9 S i N M R spectra both imply C s symmetry for this molecule. Complex 4.8 is also available from the complementary reaction of 4.6 and one equivalent of «-butylsilane. This molecule is stable in solution, which implies that no amide migration is occurring. This suggests that the second hydrosilylation reaction leading to 4.7 probably proceeds at the bridging nitride rather than a ligand amido donor. This is further supported by the complementary synthesis of 4.8 from 4.6 and butylsilane. Therefore, the amide migration scenario in Scheme 4.3 is more likely in the formation of 4.7. Additionally, the stability of 4.8 over time indicates that amide migration to give an analog of 4.7 does not occur. This shows that a very subtle level of control over [ N P N J ligand amide migration is at work, in that a small and perhaps remote difference in substituents can suppress rearrangement of ancillary ligands. Page 172 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen 4.7. Attempted kinetic studies The hydrosilylation reactions are clearly more complicated than were the reactions subsequent to hydroalumination, described in Chapter 3. The sequential 3 1 P -N M R experiments presented in Section 4.4 are useful in mapping the reaction manifold for hydrosilylation of 1, but dependable kinetic data such as rate constants for processes depicted in Scheme 4.1 are difficult to obtain because of the expected dependence of three steps on silane concentration and the potentially inhibitory effects of H 2 at three steps. In addition, imido-nitrido complex 4.2 precipitates significantly from C 6 D 6 at the concentrations employed in these experiments. Finally, the caveats discussed in Section 3.9 (pgs 87 to 89) regarding reliability of rate constants determined by this method apply here as well . The order and molecularity of the hydrosilylation reaction leading to 4.1 have yet to be properly determined, and isotope effects were not detected in similar 3 1 P N M R experiments using D2-l or ^ A ^ - l . That said, interest in the N - N bond cleavage process and the availaibility of pure 4.1 suggested study of the conversion of 4.1 to 4.2 using UV-vis ib le spectroscopy. This reaction is clean as monitored by 3 I P N M R spectroscopy. In UV-v is ib le spectroscopy, absorption due to 4.1 decreases by a first-order process with A o b s = 7.00 x 10"3 s"1. This is mildly accelerated as compared to the rate reported for the analogous process in Chapter 3 (A' o b s 2 = 2.72 x 10° h"1). Anomalously, A 5 0 0 was observed to increase again after roughly four half-lives. It is possible that a competing reaction or some degradation of 4.2 is occurring, giving rise to a species with a substantial extinction coefficient at the observed wavelength. This other species has not yet been isolated. Page 123 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen 4.8. Reactions of a bifunctional silane The use of complex 4.2 as a substrate for a selective second hydrosilylation suggested an esthetically interesting experiment aimed at tethering two bimetallic complexes together by combining two equivalents of 4.2 with one equivalent of H 3 S i C H 2 C H 2 S i C H 3 , as shown in Equation 4.9. [NPN]Ta Ta [NPN] 2 H \ X HoSi 4.2 n B u B u n H 2 S i -[NPN] Ta / r U S i •2 H 5 SihU (4.9) -N \ / \ Ta [NPN] h i N — S i / S i — N H 2 \ [NPN] Ta \ N-/ / - S i H 2 , n B u Ta [NPN] After a few days in toluene solution, the presence of a new resonance at 5 -0.02 ppm in the 3 I P N M R spectrum of the mixture is observed, suggesting that a second addition reaction has taken place. Resonances reminiscent of cyclometalated complex 4.4 are also observed, necessitating workup in order to prevent further loss of product to cyclometalation. Unfortunately, the 2 9 S i { ' H } N M R spectrum of the isolated product, complex 4.9, indicates the presence of three discrete alkylsilyl groups, which suggests that the reaction in Equation 4.9 does not proceed completely. The solid-state molecular structure of this product established that the desired tethering was not achieved, and the reaction stopped at formation of [NPN]Ta(u -NSiH 2 n Bu ) (n -NSiH 2 CH 2 CH 2 SiH 3 )Ta [NPN] , 4.9. The solid-state molecular structure of complex 4.9 is shown in Figure 4.10, and selected bond lengths and angles are listed in Table 4.7. Page 124 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Figure 4.10. O R T E P drawing of the solid-state molecular structure of | N P N l T a ( L i - N S i H 2 n B u ) ( L i - N S i H 2 C H 2 C H 2 S i H 3 ) T a | N P N | , 4.9 (ellipsoids at 50% probability). S i ly l methyls and | N P N | ligand phenyl ring carbons other than ipso omitted for clarity. Table 4.7. Selected bond distances (A) , bond angles (°), and dihedral angles (°) for l N P N | T a ( [ A - N S i H 2 B B u ) ( | A - N S i H 2 C H 2 C H 2 S i H 3 ) T a | N P N | , 4 . 9 Atom A t o m Distance (A) Atom Atom Distance (A) NI N 2 2.979(4) T a l NI 2.022(7) T a l N 2 2.031(7) Ta2 NI 2.013(7) Ta2 N 2 2.053(7) NI S i l 1.710(8) N2 Si2 1.700(7) T a l Ta2 2.6759(5) T a l PI 2.744(2) T a l N3 2.076(7) T a l N 4 2.083(7) Ta2 P2 2.688(2) Ta2 N 5 2.092(7) Ta2 N6 2.078(7) Atom Atom Atom Angle O Atom Atom A t o m Angle (°) T a l NI Ta2 83.1(3) NI Ta2 N 2 94.3(3) Ta2 N2 T a l 81.9(3) N 2 T a l NI 74.7(3) T a l N I S i l 142.4(4) Ta2 N2 Si2 136.2(4) Page 125 References begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Atom Atom Atom Atom Dihedral Angle (°) T a l P I N2 T a l Ta2 Ta2 N I P2 -155.3(4) 146.30(10) Complex 4.9 is structurally similar to prototypical complex 4.3, except for the replacement of one /i-butyl group with a - C H 2 C H 2 S i H 3 moiety. Attempts to react isolated 4.9 with additional 4.2 to achieve the objective of Equation 4.8 were not successful. It is likely that tethering two complexes together is impossible due to steric restrictions - in a way, complex 4.9 is an extreme example of a bulky primary silane, and the ethyl group may not be long enough to allow the potentially reactive - S i H 3 residue to protrude from the [NPN] ligand phenyl groups that surround this complex. Solution 3 I P N M R spectroscopy shows that 4.9 is not stable toward degradation via cyclometalation to give a complex related to 4.4. It is possible that the pendant - S i H 3 group in 4.9 is facilitating this process, a hypothesis that is supported by the observation that 4.3 only undergoes cyclometalation to 4.4 in the presence of additional silane Because the /r-butylsilyl Si atoms in symmetrically silylated derivative 4.3 are ~6.4 A apart, it is not likely that H 3 S i C H 2 C H 2 S i C H 3 can silylate both dinitrogen-derived atoms of the same molecule of 1 to form a homolog to 4.3 because the ethyl bridge is too small to allow the second intramolecular hydrosilylation. The reaction between this bifunctional silane and 1 in toluene proceeds initially as for /i-butylsilane, forming complexes that are congeners of 4.1 and 4.2 by their 3 1 P { ' H } N M R spectra. Based on the spectral data for 4.3 and 4.8, resonances near 0 ppm would be expected for a second hydrosilylation, whether this second reaction was intramolecular or intermolecular. However, no such resonances are observed. Instead, resonances reminiscent of 4.4 arise in solution, and a related complex, 4.10, has been characterized crystallographically. A n O R T E P drawing of its solid-state molecular structure is shown in Figure 4.11, and selected bond lengths and angles are listed in Table 4.8. Page 126 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen Figure 4.11. O R T E P drawing of the solid-state molecular structure of |NP (Ph )NJTa (N-S iH 2 CH 2 CH 2 Si (H ) -n -N)Ta |NPN| , 4.10 (ellipsoids at 50% probability). S i ly l methyls and | N P N | ligand phenyl ring carbons other than ipso omitted for clarity. Table 4.8. Selected bond distances (A), bond angles (°), and dihedral angles (°) for | N P ( P n ) N | T a ( N - S i H 2 C H 2 C H 2 S i ( H ) - u - N ) t a | N P N | , 4.10 Atom Atom Distance (A) Atom Atom Distance (A) NI N 2 2.609(6) T a l Ta2 2.9647(8) T a l N I 2.108(9) Ta l N 2 2.113(9) Ta2 N 2 1.944(12) NI S i l 1.672(10) NI Si2 1.757(11) N2 Si2 1.720(12) T a l N3 2.062(9) T a l N 4 2.103(9) T a l PI 2.597(3) Ta2 N 5 2.094(10) Ta2 N 6 2.097(10) Ta2 P2 2.618(3) Ta2 C3 2.275(10) Page 127 References begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Atom Atom Atom Angle (°) A tom Atom Atom Angle (°) S i l N l Si2 108.0(5) N l T a l N 2 76.3(4) N l Si2 N2 97.2(4) T a l N 2 Ta2 93.8(4) ' Ta2 T a l PI 83.40(7) • T a l Ta2 P2 128.30(8) T a l Ta2 C3 92.3(3) Atom Atom Atom Atom Dihedral Angle (°) N l T a l Ta2 N 2 -15.5(6) N l T a l Ta2 P2 -16.8(3) PI T a l Ta2 CIO 6.3(2) PI T a l Ta2 P2 -9.2(3) S i l NI Si2 N 2 ' -166.6(6) This complex exhibits the cyclometalation and imide displacement of 4.4. The ' H N M R spectra of the two species are similar, showing unique aryl resonances for the cyclometalated phenyl ring and C s molecular symmetry. The conclusion is that the-S iH 3 residue st i l l available after the first hydrosi lylat ion is contributing to these rearrangements. This is supported by the observation that the similar rearrangement of 4.3 to 4.4 occurs only in the presence of additional silane. 4.9. Effects of steric bulk In comparing the complexes presented so far, it seems that those with Az-butylsilyl groups are prone to decomposition via cyclometalation but do not show [NPN] ligand amide migration. Conversely, those with phenylsilyl groups resist cyclometalation, but can undergo amide migration. Is this difference attributable to the si lyl substituents? Butyl groups are regarded as being more electron-donating, while phenyl groups are more bulky."''8 In order to explore these issues in this system, the secondary silane diphenylsilane (Ph 2 SiH 2 ) was allowed to react with 1 in hope of greatly increasing bulk while increasing electron-donating effects to a lesser extent. Monitoring the reaction via 3 I P N M R spectroscopy shows that only a transient amount of the hydrosilylated product homologous to 4.1 and 4.5 is ever observed - the decrease in the intensities of the resonances of 1 is matched by the appearance of resonances associated with complex 4.11, a species that resembles the nitrido complex 4.2 based on its ' H and 3 I P spectra. The structure of 4.11 is assigned on the basis of its ' H and 3 1 P N M R spectra by comparison to Page 128 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen complexes 4.2 and 4.6. The reaction, which takes several days to complete, is shown in Scheme 4.4. r Ph i Scheme 4.4 Although 4.11 should feature a reactive nitrido ligand, it does not react with additional primary or secondary silane. It is possible that the additional bulk of the secondary silane is compressing the | N P N | ligand phenyl groups in a manner that prevents reactions at the nitride. Resonances suggestive of complexes related to 4.3 or 4.4 are not observed, even over weeks in the presence of /z-butylsilane or phenylsilane. Although the lack of cyclometalation is significant, this complex is not a cleaner route to homologs of 4.3 because it is unreactive. The failure of complex 4.11 to react with either phenylsilane or n-butylsilane and the unfavorable time scale for its synthesis argued against use of secondary silanes. It also left unanswered the question of how substituent effects govern the rearrangements of these complexes. A more direct comparison involving the sterically demanding primary silanes fe/V-butylsilane and mesitylsilane was undertaken. Although mesitylsilane reacts with 1, there is no evidence for an addition product comparable to prototypical hydrosilylated complex 4.1. In fact, the 3 I P N M R spectrum of the reaction mixture after 24 hours contains a plethora of resonances, none of which could be tentatively identified Page 129 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen as belonging to the family of complexes described in this thesis. This reaction was abandoned after several fruitless attempts at separating these compounds. By contrast, /-butylsilane appears to react with 1 via hydrosilylation, and transient resonances suggestive of a /-butyl analog of 4.1 can be observed in the 3 1 P N M R spectrum of 2:1 reactions. However, this material appears to convert rapidly into a number of other " P - N M R active species. One of these has been isolated and characterized crystallographically. Figure 4.12 shows the O R T E P drawing of complex 4.12, which can be formulated as [NP(Pn)N|Ta(u-N-SiH 2 'Bu)( 'BuS | i (H)N=) : ?a |NPN| . Selected bond lengths and angles are listed in Table 4.9. F igure 4.12. O R T E P drawing of the solid-state molecular structure of 4.12, |NP(p'h)N|Ta(u-N-SiH2 lBu)('BuSi(H)N=7ra[NPN| (ellipsoids at 50% probability). S i ly l methyls and | N P N ] ligand phenyl ring carbons other than ipso omitted for clarity. Page 130 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Table 4.9. Selected bond distances (A), bond angles (°), and dihedral angles (°) for |NP(Ph)NJTa(u J -N-SiH 2 l Bu)( l BuSi(H)N=)Ta[NPN], 4.12. Atom Atom Distance (A) Atom Atom Distance (A) T a l Ta2 2.9517(10) T a l N l 1.977(4) Ta2 N l 2.048(5) N l S i l 1.729(5) Ta2 N 2 1.807(5) N 2 Si2 1.759(5) Si2 N 6 1.718(5) T a l PI 2.4817(17) Ta2 C 9 2.274(5) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) T a l N3 N l T a l Ta2 N 4 94.31(19) 106.49(19) T a l N2 N l Si 120.5(3) Ta2 N 5 108.3(2) Atom Atom Atom Atom Dihedral Angle (°) PI N l T a l T a l Ta2 Ta2 N l C9 179.21(19) 172.7(2) This complex exhibits a combination of cyclometalation and amide migration. The bulky /-butyl groups have engaged each atom of the dinitrogen fragment, but it appears that the /-butylsilylimides are too sterically bulky to remain near the T a 2 N 2 core. Complex 4.12 shows unprecedented rearrangement of the dinitrogen and | N P N ] ligand components, and the | N P N | ligand that did not undergo cyclometalation has lost phosphine binding to Ta2. Further efforts to optimize the synthesis of stable congeners of the symmetric Z>/s(silylimido) complex 4.3 using bulky silanes were discontinued. The solution N M R data indicate C, symmetry in agreement with the solid state, and the cyclometalated ring phenyl protons are also featured. Two r-butyl resonances, each integrating to nine protons, are apparent. The 3 I P N M R spectrum features two singlets at 5 -31.4 and 84.3 ppm, which is the largest difference in 3 ' P chemical shift yet observed in a derivative of 1. The 3 1 P chemical shift of lithiated ligand precursor | N P N | L i 2 ( T H F ) 2 is 8 -37.7 ppm, 3 0 and resonances near 70 ppm are observed for the cyclometalated species reported herein. Thus, it seems likely that these two resonances can be assigned as P2 (uncoordinated phosphine) and PI (phosphine bound to cyclometalated phenyl substituent) of Figure 4.12, respectively. Page 131 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen 4.10. Summary and conclusions Cleavage and functionalization of molecular nitrogen under mild conditions continues to be a major chemical challenge. 3 9 This Chapter describes a system that demonstrates both of these processes. Hydrosilylation of most substrates requires catalysts, and yet hydrosilylation of 1 is facile. This reaction proceeds via wel l -characterized intermediates that are isostructural with intermediates observed in the hydroboration and hydroalumination studies, suggesting that these reactions may all belong to the same family. N - N bond cleavage is observed in the hydroboration and hydroalumination products, but the nitrido and imido complexes that are isolated always show unfavorable involvement of the ancillary [NPNj ligands. In contrast, the work in this Chapter shows that discrete nitrido and imido complexes with unaffected [NPNJ ligands can be isolated, characterized, and in most cases used as synthons. Selective functionalization of the new nitrido ligand arising from N - N bond cleavage is possible, and four different variants on the generalized bis-\m\do complex ( [ N P N | T a ) 2 ( L i - N S i H 2 R ) have been characterized. These complexes feature dinitrogen-derived atoms that have been functionalized in a chemically equivalent manner despite their asymmetric activation in parent complex 1. This is summarized in Scheme 4.5. Hydrosilylation ^-jj/,, N-N bond cleavage / N < s . L n SiH [ N P N ^ , \ ^ J a [ N P N ] . ^ [ N P N ] T a < ^ ^ T a [ N P N ] 1 • / } , ^ - N / N H N H I L n Si 4.1, L n = n B u H 2 4.5, L n = P h H 2  / j4.2, L n = n B u H 2 4-6, Ln = P h H 2 4.11, L n = P h 2 H S ' L n Second 4.3, L n = L n ' = n B u H 2 | hydrosilylation " L " = „ L"' = P ^ / \ L„ 'SiH 4.8, L n = B u H 2 , !_„• = P h 2 H [ N P N ] T a ^ - ; T a [ N P N ] ^ — M i l M J I y l U [ l 1 l 1 1 J 4.10, Ln = n B u H 2 , N " H Ln' = S i H 2 C H 2 C H 2 S i H 3 I 4.3 S iL n Scheme 4.5 Page J32 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Despite their stability relative to complexes in Chapters 2 and 3, these complexes can undergo undesirable [NPN] ligand rearrangements via two pathways: ortho-metalation of a phosphine phenyl substituent for n-butylsilyl derivatives and [NPN] ligand amide migration for phenylsilyl derivatives. The former depends on availability of additional silane. This indicates that a small measure of control over these rearrangements is available via manipulation of the silyl substituents, but how this works remains unclear. Experiments aimed at introducing silanes with greater steric bulk into this system did not lead to homologs of 4.3. Complete structural characterization of complexes 4.1 and 4.2 confirms the important role of H 2 elimination in the N - N bond cleavage event. This reaction pattern is conserved for all the derivatives of 1 prepared by addition across the Ta-N, it bond. Page 133 Rejerences begin on Page 144 4.11. Experimental Section. Chapter 4 - Hydrosilylation of coordinated dinitrogen 4.11.1 General Considerations. Unless otherwise stated, general procedures were performed according to Section 2.12.1. 4.11.2 Reagents. The primary silanes /-butylsilane, 6 0 mesitylsilane, 6 1 /z-butylsilane, and phenylsilane 6 2 were prepared by literature methods. Diphenylsilane and H 3 S i C H 2 C H 2 S i H 3 were purchased from Aldr ich and distilled under nitrogen prior to use. 4.11.3 Synthesis, Characterization, and Reactivity of Complexes. Synthesis of [NPN]Ta(H)((*-H)2(|ir^1:T|2-N2-SiH2nBu)Ta[NPN], 4.1 To a stirred solution of 758 mg (0.601 mmol) 1 in 30 m L toluene was added 53.2 mg (0.603 mmol, ~1 equiv) of butylsilane in roughly 3 m L toluene in a glove box. The flask was stored for 24 h in a - 6 0 ° C freezer, after which the dark brown solution had turned red-orange. Solvent was removed under vacuum, leaving a pinkish residue which was triturated under pentane. The resulting precipitate was recovered on a glass frit, yielding 729 g (0.593 mmol, 98.6% yield) 4.1. ' H N M R ( C V D 8 , -60 °C, 400 M H z ) : 5 -0.17, -0.09, 0.01, 0.02, 0.04, 0.40 (s, 3 H each) 0.03 (s, 6 H , total 24 S i C / / 3 ) , 0.72, 1.05, 0.46, 1.23, 0.78, 1.29, 1.20, and 1.81 ( A M X , 1H each, S i C / / 2 P ) , 5.05 and 3.97 (d, 2 J H H 11.7 Hz , 1H each, N S i / / 2 B u ) , -0.25 (br, 2 H , S i C / / 2 C H 2 - ) , 0.74 (br m, 2 H , - C H 2 C / / 2 C H 2 - ) , 1.05 (br, 2 H , - C H 2 C / / 2 C H 3 ) , 0.81 (d, 3 H , C H 2 C / / ( ) , 7.13, 7.01, 7.79, 7.20, 7.33, 7.42, 7.07, 7.13, 7.225, 6.93, 7.32, 7.50, 7.24, 7.34, 7.19, and 7.24 (overlapping m, PPh- / / and NPh- / / ) 8.342 and 7.77 (m, 2 H each, PPh-o-/ /) , 11.21 and 11.23 (ddd, 1H each, J H b H h = 3.8 Hz , 2 J H b H I = 5.52 Hz , 2 J H P = 11-7 Hz , Ta//Ta) 14.27 (dd, V H P = 17.99 H z , 7H H= 5.52 Hz , 1H, Ta-H^). 1 3 C N M R ( C V D 8 , -60 °C, 100.61 M H z ) : 0.29, 0.41, 0.49, 1.11, 2.21, 2.38, 3.27, 3.93 (s or d, S i C H 3 ) , 14.98, 23.61, 26.20, 32.61 (br s, S i C H 2 P ) , 1.81 (s, S i C H 2 C H 2 - ) , 34.81 (s, - C H 2 C H 2 C H 2 - ) , 18.54 (s, - C H 2 C H 2 C H 3 ) , 14.09 (s, C H 2 C H 3 ) , 127.47, 121.82, 128.52, 122.19, 129.46, 134.03, 121.33, and 134.93 (P (C 6 H 5 ) and N ( C 6 H 5 ) . 3 1 P { ' H } Page 134 References begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen N M R ( C 7 D 8 , -60 °C, 161.97 M H z ) : 8 23.32 (d, 7 P P = 17.83 Hz) , 8.91 (d, 7 P P = 17.83 Hz). 2 9 S i N M R ( C 7 D 8 , -60 °C, 79.5 M H z ) 8 -14.96 (s, NS/H 2 Bu) 11.17 (d, 2 J P S i = 11.3 Hz), 8.98 (d, 2 J P S , = 9.6 Hz) 8.79 (d, 2 J P S l = 8.7 Hz) , 8.49 (d, 2 J P S i = 14.3 Hz). U V - V i s : X m a x = 500 nm, 8 = 577 M ' 1 c m 1 . Ana l . Ca l c ' d for C 5 2 H 7 ( i N 6 P 2 S i 5 T a 2 : C 46.28; H 5.68; N 6.23. Found: C 45.98; H 5.45; N 6.48. Synthesis of l5N2-4.l. A solution of ^ A ^ - l was treated in a manner similar to the preparation of 4.1. ' ^ N N M R ( C V D 8 , -60 °C, 40 M H z ) 8 -163.5 (d, ' J N N = 16.6 Hz) -52.5 (dd, 2 J P N = 26.4 Hz , % N = 16.6 Hz). Additional coupling of 26.4 H z was observed in the 8 23.32 ppm resonance in the 3 ' P { ' H } spectrum, as was an additional coupling of 5.2 Hz in the 8 -14.96 ppm resonance of the 2 9 S i { ' H } spectrum. Synthesis of [NPN]Ta(H)(u.-N)(^-N-SiH2nBu)Ta[NPN], 4.2. A red/orange 15 m L toluene solution of 4.1 (231 mg, 0.188 mmol) was left at 15°C in a glove box for 36 h, after which the solvent was removed under vacuum, leaving a yellow-brown residue which was triturated under hexanes. Fine yellow-white needles of 4.2 were recovered on a glass frit (212 mg, 0.173 mmol, 92 % yield). ' H N M R ( C 6 D 6 , 30 °C, 400 M H z ) : 8 -0.12, 0.06, 0.17 and 0.38 (s, 6 H each, 24H total, SiC/7 3 ) , 1.00, 1.26, 1.22, and 1.43 (d, 2 H each, SiC/7 2 P), 4.38 (b, 2 H , NSi/7,Bu), -0.18 (br, 2 H , S i C / 7 2 C H 2 - ) , 0.78 (br m, 2 H , - C H 2 C / 7 2 C H 2 ) , 1.11 (br, 2 H , - C H 2 C / 7 2 C H 3 ) , 0.86 (d, 3 H , C H 2 C / 7 ? ) , 7.05, 7.19, 7.38, 7.47, 6.94, 6.98, 7.12, 7.26, 7.36, and 7.53 (overlapping m, PPh-/7and NPh-H) 7.85 and 7.30 (m, 2 H each, PPh-o-/7), 17.27 (d, 2 7 H P = 41.14 Hz , I H , Ta-H,). 1 3 C N M R (C 6D 6> 30 °C, 100.61 M H z ) : 8 0.37, 0.98, 1.29, 1.41, (s, S i C H 3 ) , 22.12 and 31.45 (br s, S i C H 2 P ) , 2.88 (s, S i C H 2 C H 2 - ) , 25.47 (s, - C H 2 C H 2 C H 2 - ) , 14.39 (s, - C H 2 C H 2 C H 3 ) , 22.44 (s, C H 2 C H 3 ) , 116.66, 121.58, 122.41, 127.33, 127.90, 1288.36, 129.57, 133.12 (broad overlapping resonances, P ( Q H 5 ) and N(C ( ; H,)) . 3 1 P { ' H } N M R (C 6D 6> 30 °C, 161.97 M H z ) : 8 -5.73 (b, F W H M 28 Hz) , -12.26 (b, F W H M 28 Hz). 2 9 S i N M R ( C 6 D 6 , 30 °C, 79.5 M H z ) 8 -31.47 (s, NS/H 2 Bu) 11.29 (d, 2 J P S , = 11.3 Hz) , 11.47 (d, 2 J P S i = 12.0 Hz) . Anal . Ca lc 'd for C 5 2 H 7 4 N 6 P 2 S i 5 T a 2 : C 46.35; H 5.54; N 6.24. Found: C 46.04; H 5.24; N 5.94. Page 135 References begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Synthesis of "N3-4.2 A C A solution of roughly 40 mg (0.03 mmol) /5N2-4A in a Wi lmad N M R tube was allowed to decompose overnight at 15°C in a glove box. 1 5 N N M R ( C 6 D 6 , 30 °C, 40 M H z ) 8 284.4 (b) -44.8 (d, 2 J P N = 18.7 Hz,) . Addit ional coupling of 18.7 H z was observed in the 8 -12.26 ppm resonance in the 3 I P { ' H } spectrum, as was an additional coupling of 4.4 H z in the 8 -31.47 ppm resonance of the 2 9 S i { ' H } spectrum. Synthesis of ([NPN]Ta)2(u.-N-SiH2nBu)2, 4.3 To a stirred solution of 488 mg (0.387 mmol) 1 in 10 m L toluene was added 75.4 mg (0.855 mmol, 2.2 equiv) of butylsilane in roughly 2 m L toluene in a glove box. The dark brown solution turned dark red over the course of 36 h and solvent was removed under vacuum, leaving a red/purple residue that was triturated under hexanes. The resulting precipitate was recovered on a glass frit and dissolved in roughly 10 m L 1:1 benzene/hexamethyldisiloxane, and 323 mg (0.245 mmol, 63% yield) of pure crystalline 4.3 was recovered after slow evaporation. Alternately, 21 mg (0.23 mmol) butylsilane was added dropwise to a stirred 20 m L toluene solution of 308 mg (0.229 mmol, 1 equiv) 4.2. The solution was stirred and turned deep red over four days. Solvent was evaporated and the dark solids were triturated under pentane, giving 271 mg 4.3 (0.201 mmol, 88% yield). ' H { 3 I P } N M R ( C A , 30 °C, 400 M H z ) : 8 -0.01, 0.40, (s, 12H each, SiC/7 3 ) , 1.60 and 1.71 ppm (d, 4 H each, 2 J H H = 14.1 Hz, SiC/7 2 P), -0.31 (br, 2 H , S i C / 7 2 C H 2 - ) , 0.66 (br m, 2H , - C H 2 C 7 / 2 C H 2 - ) , 1.10 (br, 2 H , - C H 2 C / 7 , C H 3 ) , 0.856 (d, 3 H , C H 2 C / / , ) , 4.91 (s, 2H , Si/7 2) 6.941 (d, 4 H , p-hPhN), 7.11 (dd, 8 H , m-HPhN), 6.52 (d, 8 H , m-HPhN), 7.12 (d, 2 H , p-HPhP), 7.20 (dd, 4 H , m-HPhP) and 7.65 (d, 2H , o-HPhF). 3 1 P { ' H } N M R ( C A , 30 °C, 161.97 M H z ) : 8 0.08 ppm (s). 2 9 S i N M R ( C A , 30 °C, 79.5 M H z ) 8 8.70, (d, 2 J r e i = 7.0 Hz), -1.74 (dd, % S l = 4.1 Hz, dd, 2 J F S i = 3.9 Hz) Anal . Ca lc 'd for C ^ N ^ S i J a , : C 46.91; H 5.91; N 5.86. Found: C 47.10; H 5.95; N .6.06. Mass Spec (EI /MS) M / z 1383.26(100%). Synthesis of "N2-43. A solution of l5N2-\ was treated in a manner similar to the preparation of 4.3. 1 5 N N M R ( C A , 30 °C, 40 M H z ) 8 -23.4 (dd, J P N = 5.1 Hz, 11.4 Hz). Additional couplings of 5.1 Page 136 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen and 11.4 H z were observed in the 3 1 P { ' H } spectrum. The 8 -1.74 ppm resonance of the 2 9 S i { ' H } spectrum was rendered broad by an unresolvable coupling to ^ N . I I Synthesis of [NP(Ph)N]Ta(nBuSiN-fi-Si(H)"Bu-ji-N)Ta[NPN], 4.4 To a stirred solution of 235 mg (0.186 mmol) 1 in 15 m L toluene was added 65.5 mg (0.744 mmol, 4 equiv) of butylsilane in roughly 2 m L toluene in a glove box. The dark brown solution turned red initially and then dark brown over the course of 4 days. Solvent was removed under vacuum, leaving a brown residue that was triturated under hexanes. The resulting precipitate was recovered on a glass frit and rinsed with minimal hexanes to afford 112 mg (0.078 mmol, 42% yield) of 4 .4 . In the assignment of the ' H N M R spectrum, the protons on the metalated phenyl ring are labeled a , p\ 8 and y, with the proton nearest the metalated carbon labeled a . ' H f ' P } N M R ( C 7 D 8 , 30 °C, 400 M H z ) : 8 -0.41, 0.11, 0.14, 0.29 (s, 6 H each, S i C / / 3 ) , 0.68, 0.91, 1.31, 1.46 (d, 2 H each, S i C / / 2 P ) , 3.37 (s, 2 H , S i A / / 2 ) , 4.62 (s, I H , S i B / / ) , 0.49 (m, overlapping, 2H , S i A C / / 2 C H 2 - ) , 1.82 (br m, 2 H , S i A C H 2 C / / 2 C H 2 - ) , 1.04 (br, 2 H , S i A C H 2 C H 2 C / / 2 C H 3 ) , 1.30, (d, 3 H , S i A C H 2 C H 2 C H 2 C / / ? ) , 0.13 (m, overlapping, 2 H , S i B C / / 2 C H 2 - ) , 1.43 (br m, 2 H , S i B C H 2 C / / 2 C H 2 - ) , 1.78 (br, 2 H , S i B C H 2 C H 2 C / / 2 C H 3 ) , 1.36, (d, 3 H , S i B C H 2 C H 2 C H 2 C / / 5 ) , 7.61 (dd, I H , H J , 7.21 (d, 1H, Hp), 7.21 (d, I H , H Y ) , 6.38 (d, 1H, H s ) , 7.11, 7.72, 7.91 (P-C6H5) 6.89, 6.95, 7.05, 7.13, 7.20, 7.22 (d, t, 23H total, some overlap with solvent, N -C 6 / / 5 ) . 3 I P { ' H } N M R ( C 6 D 6 , 30 °C, 161 .97MHz) : 8 24.23 (s), 71.15 (s). Anal . Calc 'd for C 5 6 H 8 2 N 6 P 2 S i 6 T a 2 : C 46.98; H 5.77; N 5.87. Found: C 47.12; H 5.91; N 5.89. Synthesis of l 5 N 2 - 4 . 4 A solution of l5N2-l was treated in a manner similar to the preparation of 4 .4 . 3 I P { ' H } N M R (C 6D 6, 30 °C, 161.97 M H z ) : 8 24.23 (s), 71.15 (d, 2 J B N = 9.9 Hz). 1 5 N N M R (CA. 30 °C, 40 M H z ) 8 -17.5 (d, 2 J P N = 9.9 Hz), 233.4 (s). 3 1 P N M R spectroscopic investigation of the reaction of 1 with n-butylsilane. A 9" Wi lmad N M R tube was charged with 45.3 mg 1 in roughly 0.8 m L C 6 D 6 and a sealed glass capillary tube containing neat P(OMe) 3 as an internal reference. The tube Page 137 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen was sealed with a 5 mm rubber septum and wrapped with ParaFilm laboratory f i lm and inserted into the probe of a Bruker A V A - 4 0 0 N M R spectrometer. The spectrometer was programmed to observe consecutive sets of ' H { 3 I P} and 3 I P { ' H } spectra every 15 minutes for 25 hours. After initial spectrometer calibration was performed, the sample was ejected and 14 mg neat n-butylsilane (roughly 4.5 equivs) in approximately 0.25 m L QDf, w a s added as a bolus through the septum using a 20-gauge hypodermic needle. The reagents were mixed by brief inversion of the tube before the sample was returned to the probe and automated acquisition was begun. Individual resonances were integrated with respect to the internal standard. Similar experiments using D2-l and '5N2-\ were also conducted. Synthesis of [ N P N l T a ^ ^ - ^ ^ ^ - T i ^ T i ' - N N S i H ^ ^ T a t N P N ] , 4.5 To a stirred solution of 441 mg (0.350 mmol) 1 in 25 m L toluene was added 37.9 mg (0.35 mmol, 1 equiv) of phenylsilane in roughly 2 m L toluene in a glove box. The flask was stored for 24 h in a freezer, after which the dark brown solution had turned red-orange. Solvent was removed under vacuum, leaving a pink residue that was triturated under hexanes and left overnight. The resulting red crystalline material was recovered on a glass frit, yielding 488 mg (0.345 mmol, 99% yield) 4.5. 0.5 equivs cocrystallized hexanes could not be removed from the solid under full vacuum, and its presence was confirmed by N M R and X-ray crystallography. ' H N M R ( C 7 D 8 , -60 °C, 400 M H z ) : 8 -0.12, -0.07, -0.02, 0.01, 0.02, 0.05, 0.09, 0.52 (s, 3 H each, total 24 SiC7/ 3 ) , 0.47, 0.81, 1.25, 1.29, 1.37, 1.58, 1.74, and 1 .89 ( A M X , I H each, S i C / / 2 P ) , 4.62 and 5.38 (d, 2 J H H 10.9 Hz , I H each, NSi / / 2 Ph) , 6.76, (o-Q77 5-Si), 6.82, 7.11 ( Q / / 5 - S i ) , 7.38 (d, J H H = 7.3, tf-Q/75-P) 8.40 (d, J H H = 5.0, o-C6Hs-P) 6.47, 6.76, 6.82, 6.95, 7.13, 7.18, 7.20, 7.28, 7.46 (overlapping m, PPh- / /and NPh-/ / ) , 11.24 (ddd, 2 H , J H H = 10.1 and 5.24 Hz, J H P = 22.1 Hz, Ta//Ta) 14.30 (dd, 2 7 H P =18.1 Hz , 7 H H = 5.24 Hz , I H , Ta-H t ) . I 3 C N M R ( C 7 D 8 , -60 °C, 100.61 M H z ) : -0.33, 0.52, 0.80, 0.97, 2.19, 2.86, 3.85, 4.14 (s, S i C H 3 ) , 15.06, 15.39, 16.24, 17.18, 22.32, 22.05, 19.58, 20.69 (d, S i C H 2 P ) , 130.02 (o-C 6 H 5 -S i ) 132.03 and 133.60 ( o - Q H 5 - P ) 116.44, 117.86, 121.87, 122.46, 123.01, 126.31, 129.22, 130.02, 132.65, 1334.16, 136.31, 138.44, 139.89, 143.98, 145.77, 146.21, 147.04, 147.69, 152.37, 155.69, 156.81, 157.31 ( P ( Q H 5 ) and N ( Q H 5 ) . 3 I P { ' H } N M R ( C 7 D 8 , -60 °C, 161.97 Page 138 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen M H z ) : 8 23.27 (d, 7 P P = 17.8 Hz) , 8.86 (d, 7 P P = 17.8 Hz). 2 9 S i N M R ( C 7 D 8 , -60 °C, 79.5 M H z ) 8 -9.26 (s, N ^ H 2 P h ) 6.73 (d, 2 J P S l = 9.6 Hz) , 7.16 (d, 2 J P S , = 9.6 Hz) 10.68 (d, 2 J P S i = 11.3 Hz), 11.30 (d, 2 J P S , = 9.6 Hz). U V - V i s : X m a x = 500 nm, e = 619 M 1 c m 1 . Anal . Ca lc 'd for C 5 4 H 7 2 N 6 P 2 S i 5 T a 2 ( C 6 H 1 4 ) 0 5 C 48.47; H 5.64; N 5.95. Found: C 48.63; H 5.20; N 5.48. Synthesis of l5N2-4.5. A solution of l5N2-l was treated in a manner similar to the preparation of 4 . 5 . 1 : , N N M R ( C 7 D 8 , -60 °C, 40 M H z ) 8 -139,7 (d, ' J N N = 17.4 Hz) -50.5 (dd, 2 J P N = 26.1 Hz , ' J N N = 17.4 Hz). Additional coupling of 26.1 Hz was observed in the 8 23.27 ppm resonance in the 3 1 P { ' H } spectrum, as was an additional coupling of 4.9 Hz in the 8 -9.26 ppm resonance of the 2 9 S i { ' H } spectrum. Synthesis of [NPN]Ta(H)(^N)(u.-N-SiH2Ph)Ta[NPN], 4.6. A red/orange 25 m L toluene solution of 4 . 5 (488 mg, 0.345mmol) was left at 15 °C in a glove box for 36 h, after which the solvent was removed under vacuum, leaving a yellow-brown residue which was triturated under hexanes. Solid powdery 4.6 was recovered on a glass frit and rinsed with hot hexanes to remove a brown impurity (473 mg, 0.335 mmol, 97 % yield). ' H N M R (QA, 30 °C, 400 M H z ) : 8 -0.18, -0.04, 0.03 and 0.41 (s, 6 H each, 24H total, S i C / 7 3 ) , 0.98, 1.16, 1.34, and 1.48 (d, 2 H each, S i C / / 2 P ) , 4.76 (b, 2 H , NSi / / 2 Ph) , 6.48, 6.92, 7.08, 7.47, 6.94, 7.09, 7.15, 7.38, and 7.61 (overlapping m, PPh - /7 and NPh-H) 7.92 and 7.41 (m, 2 H each, PPh-o-//), 18.21 (d, V H P = 47.3 Hz , I H , Ta-H t ). 3 1 P { ' H } N M R ( Q D 6 , 30 °C, 161.97 M H z ) : 8 -5.4 (b), -12.8 (b). 2 9 S i N M R ( C 6 D 6 , 30 °C, 79.5 M H z ) 8 -38.62 (s, NS/H 2 Ph) 11.51 (d, 2 J P S i = 12.1 Hz) , 10.81 (d, 2 J P S , = 12.4 Hz). Anal . Calc 'd for C ^ H ^ N ^ S i s T a , : C 47.43; H 5.16; N 6.15. Found: C 47.80; H 5.51; N 5.94. Synthesis of ,5N2-4.6 A solution of '5N2-4.5 in C 6 D 6 was allowed to decompose overnight at room temperature. 1 5 N N M R ( C 6 D 6 , 30 °C, 40 M H z ) 8 312.2 (b) -36.9 (d, 2 J P N = 17.2 Hz,) . Additional coupling of 17.2 H z was observed in the 8 -12.8 ppm resonance in the 3 I P { ' H } spectrum, Page 139 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen as was an additional coupling of 4.1 H z in the 5 -38.6 ppm resonance of the 2 9 S i { ' H } spectrum. I I Synthesis of [NPN]Ta(n-NSiH2Ph)(fi-NSiHPh)Ta[NPN], 4.7 To a stirred solution of 312 mg (0.247 mmol) 1 in 15 m L toluene was added 55.0 mg (0.510 mmol, 2.1 equiv) of phenylsilane in roughly 2 m L toluene in a glove box. The dark brown solution turned dark red over the course of 48 h and solvent was removed under vacuum, leaving a brown residue which was triturated under hexanes. A n ochre precipitate was recovered on a glass frit, giving 205 mg (0.139 mmol, 56% yield) of analytically pure 4.7. 'H{ 3 1 P} N M R (QA, 30 °C, 400 M H z ) : 5 -0.39, -0.24, -0.23, -0.19, -0.13, -0.01, 0.22, 0.59 (s, 3 H each, S i C / / 3 ) , 1.13, 1.23, 1.28, 1.43, 1.61, 1.94, 2.29, and 2.79 ppm ( A M X , I H each, SiC/7 2 P) , 4.60 and 5.68 (d, I H each, 2 J H H = 13.3Hz, Ph5/H 2 -N), 5.59 (s, N-PhSi(H)-N), 7.54, 7.62 (d, 2 H each, 0 - Q / / 5 - P ) 9.43 (s, 2 H o - Q / / 5 - S i H 2 - N ) , 6.52, 6.69, 6.79, 6.88, 6.93, 7.04, 7.21, 7.36, 8.18 (overlapping m, PPh-/7and NPh-/7). I 3 C N M R ( C 6 D 6 . 30 °C, 100.61 M H z ) : 5 -1.56, -0.59, -0.38, 0.87, 3.30, 4.03, 4.12, 4.81 (S iCH 3 ) , 15.22, 18.14,24.22, 35.17 (CH2-P), 125.91 (N-o-C 6 H 5 Si (H)-N) 129.07 (o-C6H5-S i H 2 - N ) , 123.47, 124.44, 126.39, 127.85, 129.58, 132.48, 133.69, 137.58 ( P - Q H 5 and N -C 6 H 5 ) . 3 I P { ' H } N M R (QA, 30 °C, 161.97 M H z ) : 6 -10.46, 15.88 ppm (s). 2 9 S i N M R (QA, 30 °C, 79.5 M H z ) 5 -35.82, (dd, 2 J P S i = 12.2, 16.6 H z , N-PhSi(H)-N) 6.07 (s, [NPN] ligand) 12.93 (d, 2 J P S | = 14.8 Hz , [NPN] ligand), 17.07 (d, 2 J P S l = 14.8 Hz , [NPN] ligand), 18.30 (d, 2 J P S , = 20.1 H z , [NPN] ligand), 49.86 (d, 2 J P S i = 41.0 Hz , P h S M 2 - N ) . Anal . Calc 'd for C 6 0 H 7 4 N 6 P 2 S i 6 T a 2 : C 48.90; H 5.20; N 5.70. Found: C 48.63; H 5.20; N 5.48. Synthesis of 15N2-4.7. A solution of ^ A ^ - l was treated in a manner similar to the preparation of 4.7. 3 1 P { ' H } N M R (QA, 30 °C, 161.97 M H z ) : 6 -10.46 (d, J P N = 17.86 Hz). 15.88 ppm (s). 1 5 N N M R (CA;, 30 °C, 40 M H z ) 6 -61.21 (d, J P N = 17.86 Hz). -35.84 (s). ). 2 9 S i N M R (CA;. 30 °C, 79.5 M H z ) 6 -35.82, (ddd, 2 . l P S i = 12.2, 16.6 Hz , ' J N S i = 10.5 Hz , N-PhSi(H)-N), 49.86 (dd, 2 J P S i = 41.0 Hz , PhS7H 2-N), [NPN] ligand silyl resonances unaffected. Page 140 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen Synthesis of [NPN]Ta(u.-NSiH2 nBu)(^NSiH2Ph)Ta[NPN], 4.8 To a stirred 20 m L toluene solution of 126 mg (0.094 mmol, ~1 equiv) 4.2 was added 9 mg (0.10 mmol) phenylsilane. The stirred solution turned deep red over two days. Solvent was evaporated and the dark solids were triturated under pentane, giving 97.8 mg 4.8 (0.067 mmol, 72% yield). Alternately, complex 4.6 was combined with /z-butylsilane in Q A in an N M R tube, and the 3 1 P N M R resonances of 4.8 were observed after two days. No yield is reported in the absence of an internal standard. 'H{ 3 ' P} N M R ( Q A > 30 °C, 400 M H z ) : 8 -0.14, -0.12, 0.02, 0.10, (s, 6 H each, S i C / / 3 ) , -0.31 (br, 2 H , S i C / / 2 C H 2 -), 0.62 (br m, 2 H , - C H 2 C / / 2 C H 2 - ) , 1.24 (br, 2 H , - C H 2 C / / 2 C H 3 ) , 0.856 (d, 3 H , C H 2 C / / V ) , 1.02, 1.16, 1.31, and 1.64 ppm (d, 2 H each, S i C / / 2 P ) , 4.78 (s, 2 H , PhSi / / 2 -N) , 4.41 (s, 2 H B u S i / / 2 - N ) , 6.37 (d, 2 H , o - Q / / 5 - S i ) , 7.34 and 7.49 (d, 2 H each, o - Q / / 5 - S i ) 4.91 (s, 2 H , S i / / 2 ) 6.941 (d, 4H,V-/YPhN), 7.11 (dd, 8 H , m-HPhN), 6.52 (d, 8 H , m-HPhN), 7.12 (d, 2 H , p-HPhP), 7.20 (dd, 4 H , m-HPhP) and 7.65 (d, 2 H , o-HPhP), 6.87, 6.93, 6.95, 6.97, 6.99, 7.05, 7.08, 7.23, 7.27 (overlapping m, PPh- / /and NPh-/ / ) . 3 1 P { ' H } N M R ( Q A , 30 °C, 161.97 M H z ) : 8 -1.17 ppm (d, J P P = 9.9 Hz), -0.70 (d, J P P = 9.9 Hz). 2 9 S i N M R ( Q A , 30 °C, 79.5 M H z ) 8 -11.3, (d, 2 J P g l = 8.0 Hz , PhS/H 2 -N), -1.8 (d, 2 J P S , = 8.8 Hz , Bu5 iH 2 -N) , 10.93 (d, 2 J P S l = 14.8 H z , [NPN] ligand) 12.9 (d, 2 J P S , = 12.4 Hz [NPN] ligand) Ana l . Calc 'd for C 5 8 H 8 0 N 6 P 2 S i ( ) T a 2 : C 47.92; H 5.55; N 5.78. Found: C 47.60; H 5.15; N 6.07. Synthesis of [NPN]Ta(ji-NSiH2"Bu)(M,-NSiH2CH2CH2SiH3)Ta[NPN], 4.9 To a stirred 15 m L toluene solution of 383 mg (0.284 mmol, 1 equiv) 4.2 was added 25.7 mg (0.284 mmol) ethanediylbissilane. The stirred solution turned deep red over four days. Solvent was evaporated and the dark solids were triturated under pentane, giving 289 mg 4.9 (0.202 mmol, 71% yield). ' H { 3 I P } N M R ( Q D 6 , 30 °C, 400 M H z ) : 8 0.00, 0.11, 0.38, 0.40, (s, 6 H each, S i C / / 3 ) , -0.35 (br, 2 H , S i C / / 2 C H 2 - ) , -0.12 (br m, 2 H , -C H 2 C / / 2 C H 2 - ) , 1.58 (br, 2 H , - C H 2 C / / 2 C H 3 ) , 1.02 (d, 3 H , C H 2 C / / , ) , -0.05 ( d, 2 H , N -S i H 2 C H 2 C / / 2 S i H 3 ) , 0.34 ( d, 2 H , N - S i H 2 C / / 2 C H 2 S i H 3 ) 1.63 ppm ( A M X , 8 H , S iC/7 2 P) , 3.63 (t, 2 J H H = 3.8 H z , 3 H , / / 3 C S i E t S i H 2 - N ) , 4.81 (d, 2 J H H = 3.56 H z , H 3 C S i E t S i / / 2 - N ) , 4.89 (s, 2 H , Si/7 2 Bu), 7.62 (d, 4 H , o-HPhP), 6.90, 6.92, 7.01, 7.06, 7.10, 7.12, 7.19, 7.22 (overlapping m, P P h - / / and NPh-/ / ) . 1 3 C N M R ( Q A , 30 °C, 100.61 M H z ) : 8 -2.42, 1.52, 4.85, 5.12 ( S i C H 3 ) , 25.11, 26.20 ( C H 2 - P ) 25.11, 26.20, 13.96, 3.87 (BuS\H2), 3.87 Page 141 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen ( H 3 S i C H 2 ) , 12.89 ( H 3 S i C H 2 C H 2 S i H 2 - N ) , 131.46, 131.57 (o-Ph-P), 122.44, 122.54, 123.61, 125.21, 128.10, 128.50, 128.66, 128.84 (PPh and NPh). 3 I P { ' H } N M R ( C 6 D 6 , 30 °C, 161.97 M H z ) : 5 -0.021 ppm (m). 2 9 S i - D E P T N M R ( Q D ( „ 30 °C, 79.5 M H z ) 8 -2.14 (H 3 SiEtS /H 2 -N) , -0.01 (BuS/H 2 -N) 8.08 (d, 2 J H S i = 15.7 Hz , [NPN] ligand) 8.79 (d, 2 J P S i = 17.4 H z , [NPN] ligand). H 3 S7-Et s i ly l resonance not located. A n a l . C a l c ' d for C54H 8 2 N 6 P 2 Si 7 Ta 2 : C 45.18; H 5.76; N 5.85. Found: C 45.23; H 5.91; N 5.96. Synthesis of [NP(Ph)N]Ta(N-SiH2CH2CH2Si(H)-|i-N)Ta[NPN], 4.10. To a stirred 10 m L toluene solution of 184 mg (146 mmol) 1 was added neat ethanediylbissilane (13 mg, 146 mmol , 1 equiv). The solution was allowed to sit overnight, and solvent was removed under vacuum to give a yellow-brown residue that yielded crystalline 4.10 (138 mg, 105 mmol, 72% yield) after trituration under hexanes and 24 hours at - 6 0 ° C in a glove box freezer. 'H{ 3 1 P} N M R ( C 7 D 8 , 30 °C, 400 M H z ) : 8 -0.80, -0.08, 0.13, 0.47 (s, 6 H each, 24H total, SiC/7,), 0.68, 0.92, 1.34, 1.76 (d, 2 H each, -CH2P), 3.52 (s, 2 H , Si/7 2 ), 4.11 (s, I H , Si/7) 7.74 (d, 2 H , o-Q77 5-P), 7.72 (dd, I H , H J , 7.03 (d, I H , Hp), 6.47 (d, I H , H Y ), 6.79 (d, I H , H s ) 6.82, 6.84, 6.91, 7.02, 7.14, 7.31, 7.44 (d, t, overlapping with solvent resonances, Q77, -N and C 6 /7 5 -Ph). 3 I P { ' H } N M R ( C 7 D 8 , 30 °C, 400 M H z ) : 8 21.7 (s), 72.8 (s). Elemental analysis was not obtained. Synthesis of [NPN]Ta(u.-N)(u.-NSiHPh2)Ta[NPN], 4.11 To a 15 m L toluene solution of 268 mg (212 mmol) 1 was added 39.3 mg (213 mmol, ~1 equiv) P h 2 S i H 2 in roughly 2 m L toluene. The mixture was agitated briefly and allowed to stand in a glove box at ambient temperature. The 3 1 P N M R spectrum of a portion of the solution withdrawn after 2 weeks indicated conversion to a new product, and solvent was removed under vacuum. Brown residues were triturated under hexanes and 191 mg (131 mmol, 62 % yield) of yellow-white powdery 4.11 were isolated on a frit after rinsing with minimal hexanes. ' H { 3 I P } N M R ( C 7 D 8 , 3 0 °C, 400 M H z ) : 8 -0.28, -0.03, 0.16, 0.31 (s, 6 H each, 24H total, SiC/7), 0.84, 0.92, 1.36, 1.49 (d, 2 H each, -C/7 2 P), 3.49 (s, I H , Si/7) 7.83, 7.90 (d, 2 H each, o-Q77 5-P), 6.85, 6.92, 7.14, 7.33, 7.38, 7.41 (d, t, overlapping with solvent resonances, Q / 7 5 - N and C 6 /7 5 -Ph), 6.43, 7.11, 7.34 ( d, t, 10H total, C 6 /7 5 Si) . 3 1 P { ' H } N M R ( C 7 D 8 , 30 °C, 400 M H z ) : 8 - 16.54 (vb, F W H M 36.2 Hz) . -9.47 (vb, Page 142 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen F W H M 36.8 Hz). Anal . Calc 'd for C 6 0 H 8 4 N 6 P 2 S i 5 T a 2 : C 49.58; H 5.82; N 5.78. Found: C 49.33; H 5.91 ; N 5.86. Synthesis of [NP(Ph)N]Ta(n-N-SiH2'Bu)CBuSi(H)N=TTa[NPN], 4.12. 33.7 mg (0.382 mmol) /-butylsilane was added to a stirred 10 m L toluene solution of 241 mg (0.191 mmol, 0.5 equiv) 1. The stirred solution turned red over 6 hours, and brown thereafter. Solvent was evaporated and the dark solids were triturated under hexanes. After standing overnight, X-ray quality crystals of 4.12 (101 mg, 0.706 mmol, 37 % yield) were recovered on a frit. In the assignment of the ' H N M R spectrum, the protons on the metalated phenyl ring are labeled a , p\ 5 and y, with the proton nearest the metalated carbon labeled a . ' H { 3 I P} N M R ( C 4 D g O , 30°C, 500 M H z ) : 5 -0.70, -0.67, -0.10, 0.01,, 0.02, 0.04, 0.21, 0.32 (s, 3 H each, S\CH3), 0.46, 0.86 (s, 9 H each, ( C Z / ^ C S i ) , 0.33, 0.66, 0.73, 1.12, 1.14, 1.38, 1.45, 1.80 (d, I H each, - C H 2 P ) , 3.99, 4.27 (d, 2JH H = 12.7 Hz , l B u S i H 2 ) , 4.93 (s, I H , N'BuSi(H)N), 7.46 (dd, I H , H J , 7.80 (d, I H , Hp), 8.04 (d, I H , H y ) , 5.89 (d, I H , H s ) , 8.76, 7.57, 7.24 (P-C6H5) 6.17, 6.99, 7.52, 6.89, 7.18, 7.47, 6.15, 6.86, 7.01, 6.28, 6.98, 7.09 (d, t, 20H total, N - Q / 7 5 ) . 3 1 P { ' H } N M R ( C 4 D 8 0 , 30 °C, 202 M H z ) : 5 -31.4 ppm (s) 84.3 ppm (s). Elemental analysis was not obtained. Monitoring conversion of 4.1 to 4.2 by UV-visible spectroscopy. A 1.0 cm quartz cuvette fitted with a Teflon valve was charged with a 1.59 m M toluene solution of 4.1 in a glove box. The cuvette was inserted in a Hewlett-Packard 8453 U V -visible spectrophotometer and A 5 0 0 was recorded over several hours. Page 143 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen References: (1) Fryzuk, M . D. ; Love, J. B . ; Rettig, S. J. ; Young, V . G . Science 1997, 275, 1445. (2) Basch, K ; Musaev, D. G . ; Morokuma, K . ; Fryzuk, M . D. ; Love, J. B . ; Seidel, W. W. ; Albinati , A . ; Koetzle, T. F.; Klooster, W . T.; Mason, S. A . ; Eckert, J. J. Am. Chem. Soc. 1999, 727, 523. (3) Brookhart, M . ; Grant, B . E . J. Am. Chem. Soc. 1993, 775, 2151. (4) Aki ta , M . Appl. Catal.. A. 2000, 200, 153. (5) Luo, X . - L . ; Kubas, G . J.; Burns, C. J.; Bryan, J. C ; Unkefer, C. J. J. Am. Chem. Soc. 1995, 117, 1159. (6) Kubas, G . J. Metal Dihydrogen and sigma-Bond Complexes; Kluwer Academic/Plenum: New York, 200]. (7) Crabtree, R. H . Angew. Chem., Int. Ed. 1993, 32, 789. (8) Schubert, U . Adv. Organomet. Chem. 1990, 30, 151. (9) Corey, J. Y . ; Braddock-Wilking, J. Chem. Rev. 1999, 99, 175. (10) Lichtenberger, D . L . ; Rai-Chaundhuri, A . J. J. Am. Chem. Soc. 1990, 7/2, 2492. (11) Nikonov, G . I.; Kuzmina, L . G . ; Vyboishchikov, S. F.; Lemenovskii, D. A . ; Howard, J. A . K . Chem. - Eur. J. 1999, 5, 2947. (12) Nikonov, G . I.; Mountford, P.; Green, J. C ; Cooke, P. A . ; Leech, M . A . ; Blake, A . J.; Howard, J. A . K . ; Lemenovskii, D . A . Eur. J. Inorg. Chem. 2000, 1917. (13) Nikonov, G . I.; Mountford, P.; Ignatov, S. K . ; Green, J. C ; Leech, M . A . ; Kuzmina, L . G . ; Razuvaev, A . G . ; Rees, N . H . ; Blake, A . J.; Howard, J. A . K . ; Lemenovskii, D . A . J. Chem. Soc, Dalton Trans. 2001, 2903. (14) Nikonov, G . I. Organometallics 2003, 22, 1597. (15) Magnus, P. D. ; Sarkar, T.; Djuric, S. In Comprehensive Organometallic Chemistry; Wilkinson, G . , Abe l , E . W. , Stone, F. G . A . , Eds.; Pergamon Press: Oxford, 1982; V o l . 7 . (16) Parks, D. J.; Piers, W. E . ; Yap , G . P. A . Organometallics 1998, 77, 5492. (17) Ojima, I. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z . , Eds.; Wiley: New York, 1989. (18) Speier, J. Adv. Organomet. Chem. 1979, 17, 407. (19) Lappert, M . F.; Scott, F. P. A . J. Organomet. Chem. 1995, 492, C l l . (20) Lewis, L . N . ; Stein, J.; Gao, Y . ; Colborn, R. E. "Synthesis and Activity of Pt Catalysts Used in the Silicones Industry," G E Polymer and Inorganic Systems Laboratory, 1996. (21) Chalk, A . J.; Harrod, J. F. J. Am. Chem. Soc 1965, 87, 16. Page 144 Rejerences begin on Page 144 Chapter 4 - Hydrosilylation of coordinated dinitrogen (22) Harrod, J. F. ; Chalk, A . J. 7 Am. Chem. Soc. 1964, 86, 1776. (23) Harrod, J. F. ; Chalk, A . J. J. Am. Chem. Soc. 1965, 87, 1133. (24) Harrod, J. F. ; Chalk, A . J. 7 Am. Chem. Soc. 1966, 88, 3491. (25) Seitz, F.; Wrighton, M . S. Angew. Chem.. Int. Ed. 1988, 27, 289. (26) Wakatsuki, Y . ; Yamazaki, N . ; Nakano, Y . ; Yamamoto, J. J. Chem. Soc, Chem. Comm. 1991, 703. (27) Schroeder, M . A . ; Wrighton, J. J. Organomet. Chem. 1977, 128, 345. (28) Duckett, S. B . ; Perutz, R. N . Organometallics 1992,11, 90. (29) Gountchev, T. I.; Til ley, T. D. Organometallics 1999,18, 5661. (30) Fu, P. F. ; Brard, L . ; L i , Y.; Marks, T. J. J. Am. Chem. Soc. 1995, 777, 7157. (31) Molander, G . A . ; Julius, M . J. J. Org. Chem. 1992, 67, 6347. (32) Til ley, T. D. Acc. Chem. Res. 1993, 26, 22. (33) Glaser, P. B . ; Til ley, T. D . J. Am. Chem. Soc 2003, 725, 13640. (34) Rubin, M . ; Schweir, T.; Gevorgyan, N . J. Org. Chem. 2002, 67, 1936. (35) Song, Y . S.; Yoo , B . R.; Lee, G. H . ; Jung, I. N . Organometallics 1999,18, 3109. (36) Lambert, J. B . ; Zhao, Y . ; W u , H . W. J. Org. Chem. 1999, 64, 2729. (37) Schmeltzer, J. M . ; A . , P. L . ; Stewart, M . P.; Buriak, J. M . Langmuir 2002, 78. (38) Chauviere, G . ; Corriu, R. J. P.; Henner, B . J. L . J. Organomet. Chem. 1975, 86, C I . (39) Heine, A . ; Herbst-Irmer, R.; Stalke, D . J. Chem. Soc, Chem. Comm. 1993, 1729. (40) Becker, B . ; Corriu, R. J. P.; Guerin, C ; Henner, B . J. L . 7. Organomet. Chem. 1989, 369, 147. (41) Boyer, J. ; Breliere, C ; Carre, F.; Corriu, R. J. P.; Kpoton, A . ; Poirier, M . ; Royo, G . ; Young, J. C. 7. Chem. Soc, Dalton Trans. 1989, 43. (42) Boudin, A . ; Cerveau, G . ; Chuit, C ; Corriu, R. J. P.; Reye, C. Angewandte Chemie, 1986, 98, 472. (43) Kost, D . ; Kalikhman, I.; Krivonos, S.; Stalke, D . ; Kottke, T. 7. Am. Chem. Soc. 1998, 720, 4209. (44) Klaui , W. ; Mocigemba, N . ; Weber-Schuster, A . ; Be l l , R.; Frank, W. ; Mootz, D. ; Poll , W. ; Wunderlich, H . Chem. - Eur. 7 2002, 8, 2335. (45) Kojima, M . ; Azuma, S.; Hirotsu, M . ; Nakajima, K . ; Nonoyama, M . ; Yoshikawa, Y . Chem. Lett. 2000, 482. (46) Orpen, A . G . 7. Chem. Soc, Dalton Trans. 1980, 2509. (47) Mason, J . ; Larkworthy, L . F . ; Moore, E . A . Chem. Rev. 2002, 702, 913. (48) Mason, J. Chem. Rev. 1981, 81, 205. Page 145 References begin on Page 144 Chapter 4 - Hydrosilylation oj coordinated dinitrogen (49) Fryzuk, M . D. ; MacKay , B . A . ; Patrick, B.O.J. Am. Chem. Soc. 2003, 125,3234. (50) Fryzuk, M . D. ; Johnson, S. A . ; Patrick, B . O.; Albinati , A . ; Mason, S. A . ; Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960. (51) Kawaguchi, H . ; Matsuo, T. Angew. Chem., Int. Ed. 2002, 41, 2792. (52) Clentsmith, G . K . B . ; Bates, V . M . E. ; Hitchcock, P. B . ; Cloke, F. G . N . J. Am. Chem. Soc. 1999, 727, 10444. (53) Gountchev, T. I.; Til ley, T. D. J. Am. Chem. Soc. 1997, 779, 12831. (54) Chisholm, M . Reactivity of Metal-Metal Bonds; American Chemical Society: Washington, D. C , 1980. (55) Wigley, D . E . Prog. Inorg. Chem. 1994, 42. (56) Dubberley, S. R.; Ignatov, S. K . ; Rees, N . H . ; Razuvaev, A . G . ; Mountford, P.; Nikonov, G . I. J. Am. Chem. Soc. 2003, 725, 642. (57) Nikonov, G . I. Angew. Chem., Int. Ed. 2001, 40, 3353. (58) Tolman, C . A . Chem. Rev. 1977, 77, 313. (59) Gambarotta, S. J. Organomet. Chem. 1995, 500, 117. (60) Markl , G . ; Hollriegl, H . ; Schlosser, W . J. Organomet. Chem. 1984, 260, 129. (61) Weidenbruch, M . ; Schafer, A . ; Rankers, R. J. Organomet. Chem. 1980, 195, 171. (62) Finholt, A . E . ; Bond, A . C , Jr.; Wilzbach, K. E. ; Schlesinger, H . I. J. Am. Chem. Soc. 1947, 69, 2692. Page 146 Rejerences begin on Page 144 Chapter 5 Synopsis and extensions 5.7. Synopsis of reactions The work in this thesis describes several examples of a new reaction for coordinated dinitrogen, namely the addition reaction between 1 and hydrides of boron, aluminum, and sil icon. These are the first reports of 1,2-addition reactions across a coordinated dinitrogen l igand. ' 2 In all cases, a new N - E bond is formed (E = B , A l , or Si) along with a new hydrido ligand on tantalum. Because alkylboron and alkylaluminum hydride reagents are known to react with TC bonds of other substrates, it is possible that they react with 1 in this fashion because the H O M O of 1 has TC character. Although the reactivity of silanes towards 1 was in some ways anticipated by the literature,3"5 the facile hydrosilylation of 1 is remarkable in that hydrosilylation of olefins and other organic substrates generally requires catalysis. The reaction products following hydroboration and hydrosilylation of 1 have been unambiguously characterized, and spectral data for the analogous complex resulting from hydroalumination of 1 indicates these reagents all react with 1 to give the same type of complex. Therefore, it appears that 1 is uniquely predisposed to undergo this type of reaction. In all cases, the first product of the addition reaction is unstable toward reductive elimination of its bridging hydride ligands as H 2 , and the N - N bond of the functionalized dinitrogen unit is cleaved as shown in Scheme 5 .1. This was completely unexpected, as 1 withstands weeklong heating to 110 °C without undergoing loss of H 2 and N - N bond cleavage. The Lewis acid-base adducts of 1 are also generally stable to heating. Clearly, Page 147 Rejerences begin on Page 159 Chapter 5 - Synopsis and Extensions chemical modification of 1 by addition reactions as shown in this work is a requirement for the observed reductive elimination of H 2 and subsequent N - N bond cleavage. H-E addition N-N bond cleavage / N . x H - E L n I N P N J T a ^ ^ J a [ N P N ] . ^ [NPN]Ta<^ \ a [ N P N ] 1 + / J . ^ N *• / N H N H J L E ' L N E L n E = B ( C 8 H 1 8 ) , B C y 2 , B ( H ) C 6 H 1 3 , A I 'Bu 2 , S i H 2 " B u , S i H 2 P h , S i H P h 2 , S i H 2 C H 2 C H 2 S i H 3 , S i H 2 ' B u Scheme 5.1 N - N bond scission gives rise to nitrido ligands which can be reactive. In the hydroboration study, some [NPN] ligand activation is induced by these new ligands. The use of 2 9 S i N M R spectroscopy and the isolation of an intermediate in the conversion of 2.1 to 2.2 helped to establish that ligand silyl group migration to the new nitride ligand occurs. This is further confirmed by the synthesis of a homologous series of compounds prepared using other hydroboration reagents. In the hydrosilylation study, the nitrido complexes are isolable and amenable to additional hydrosilylation reactions. This allows preparation of four complexes in which the dinitrogen-derived N atoms, activated asymmetrically in 1, are each functionalized by silylation. It is also possible to selectively introduce a variety of alkyls i lyl groups into these complexes. In general, the nitride complexes undergo [NPN] ligand amide migration or activation by cyclometalation of phosphine phenyl groups. Some of the hydrosilyated complexes are reasonably stable in the absence of additional silane. Hydroborated complexes 2.3 and 2.6 eliminate benzene from an [NPN] ligand-derived phenylimido group. Hydroaluminated complex 3.1 eliminates an alkene in a process mediated by the aluminum atom. The change from boron hydride to aluminum hydride reagents gives a qualitative acceleration of the reaction kinetics. The hydrosilylation reactions proceed at an intermediate pace as compared to the other two reactions. Page 148 Rejerences begin on Page 159 5.2. Chapter 5 - Synopsis and Extensions Mechanistic considerations and future kinetic studies The exact nature of the mechanism of H -E bond addition to 1 remains unclear. It is possible that each of these reagents reacts by different mechanisms. The group 13 reagents are widely believed to add across olefins via four-centered transition states, but hydrosilylation is almost invariably catalyzed and several mechanistic possibilities were briefly introduced in Section 4.1. Generally, an addition reaction to 1 could proceed via a four-centered transition state or by formation of an initial adduct between N, and higher-coordinate E , followed by H atom transfer onto tantalum. These two possibilities are shown in Scheme 5.2. For E = Si, the N-bound mechanism postulates a five-coordinate silicon atom as an intermediate. H-E Ph Ph V P h M e 2 s j ^ N M e 2 S j }=4 N C l ^ H ^ . f / S i M e L P H N Ph »* Ph Ph Ph Ph' Me2S/ T a T a ^ - N / /le 2Sit 4 f \ ^ l \ r ^ N p | 4-centered transition state Me. \ Ph Ph H-E = H-BR 2 , H-AIR 2, H-S iH 2 R P h P h S P — N X ^ P h > ^ iMe 2 j*"<i. r / s i M e 9 <r^\ /1". Ph N* Ph H N* / E N P h V Ph H-E M e 2 S r Ph Ph" 2Si i f / N A V Ph H ' I . E Ph Ph H atom transfer N-bound transition state Scheme 5.2 Page 149 Rejerences begin on Page 159 Chapter 5 - Synopsis and Extensions The failure of H B ( C 6 F 5 ) 2 to hydroborate 1 can be rationalized in the context of either of these possibilities. If addition occurs via a concerted four-centered process, then the reversal of B - H bond polarity and molecular dipole in H B ( C 6 F 3 ) 2 versus 9 - B B N accounts for the lack of hydroboration reactivity. If an N-bound mechanism is operant, then H B ( Q F 5 ) 2 may be too Lewis acidic to give up its hydride ligand to tantalum in the hydrogen atom transfer step. The fact that terminal tantalum hydride ligands in hydroborated complexes 2 .1 , 2 .5 , and 2.8 are transferred back onto boron as they convert to 2 .3 , 2 .6, and 2.9, respectively, suggests that this is possible, since this step is roughly the microscopic reverse of the H atom transfer step. Kinetic studies aimed at establishing the H / D isotope effect for hydroboration of 1 could indicate one of these possibilities over the other. Similar studies for hydroalumination are also possible, but the rapidity of hydroalumination could pose a challenge. Because of the amenable time scale of the hydrosilylation reaction, a kinetic analysis comparing R S i D 3 to R S i H 3 seems practical. The bottom pathway in Scheme 5.2 would likely involve a pre-equilibrium in the first step that might be evident in a kinetic analysis. Thorough kinetic study of the mechanism of N - N bond cleavage may also possible using hydrosilylated derivatives of 1. The addition products 4.1 and 4.5 convert cleanly to nitrido complexes 4 . 2 and 4 . 6 , respectively, over a few hours at room temperature. The availability of u -D , and l ; , N 2 isotopomers of these complexes suggest exploration by U V - v i s i b l e spectroscopy, but ini t ial efforts suffered from poor reproducibility and a competing reaction not indicated by 3 1 P N M R spectroscopy. 5.3. Reactions with cumulene electrophiles Cumulenes have been combined with metal-nitrogen multiple bonded systems with interesting results. The reaction of the vanadium nitrido complex [(RNAr) 3 V=NJ" with C S 2 or C O , extrudes thiocyanate or cyanate, respectively. 6 Parent dinitrogen complex 1 reacts with C 0 2 to give intractable and insoluble yellow products that have resisted characterization. The reaction between 1 and C S 2 proceeds via loss of N 2 to disassemble C S 2 as shown in Equation 5 .1 . The bridging hydrides are transferred to the carbon atom as shown by ' H / 1 3 C correlational N M R spectroscopy. A n O R T E P drawing Page 150 Rejerences begin on Page 159 Chapter 5 - Synopsis and Extensions of the solid-state molecular structure of 5.1 is shown in Figure 5.1, and selected bond lengths and angles are listed in Table 5.1. The idealized molecular symmetry of 5.1 based on this structure is C 2 , with the rotational axis passing through the bridging methylene carbon atom and the midpoint of the tantalum-tantalum internuclear axis. However, in solution only two si lyl methyl resonances and one methylene resonance are observed, and therefore this molecule appears to be undergoing a fluxional process that allows C s symmetry. Figure 5.1. O R T E P drawing of the solid-state molecular structure of ( |NPN]Ta j i -S ) 2 (L i -CH 2 ) , 5.1 (ellipsoids at 50% probability). S i ly l methyl and phenyl ring carbons other than ipso omitted for clarity. Page 151 References begin on Page 159 Chapter 5 - Synopsis and Extensions Table 5.1. Selected bond distances (A), bond angles (°), and dihedral angles (°) for ( |NPN|Tau.-S) 2(u,-CH 2), 5.1. Atom Atom Distance (A) Atom Atom Distance (A) T a l SI 2.2475(17) T a l S2 2.7590(18) Ta2 S2 2.2577(16) Ta2 SI 2.8073(17) T a l Ta2 3.0775(4) SI S2 3.582(8) T a l C I 2.200(7) Ta2 C I 2.177(7) T a l PI 2.7144(18) T a l N l 2.009(5) T a l N 2 2.073(5) Ta2 P2 2.669(4) Ta2 N3 2.038(5) Ta2 N 4 2.066(6) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) T a l C I Ta2 89.4(3) T a l SI Ta2 74.08(5) T a l S2 Ta2 74.93(5) N l T a l N 2 101.4(2) N3 Ta2 N 4 99.4(2) CI T a l PI 85.41(19) C I Ta2 P2 83.53(18) Atom Atom Atom Atom Dihedral Angle (°) PI T a l Ta2 P2 125.67(6) PI T a l CI Ta2 -122.77(19) P2 Ta2 CI T a l -120.37(19) One of the more optimistic goals laid out in Chapter 1 of this thesis was the incorporation of dinitrogen-derived N atoms into other substrates. Prototypical to-imido complex 4 . 3 is intriguing from this standpoint in that it features two equivalently functionalized N atoms that have been completely disconnected from one another. Nitrogen-carbon bond formation in reactions between C O or C 0 2 and a monometallic Cp*-supported ir idium imido complex have been reported, 7 and imides have been implicated in the copper-catalyzed aziridination of olefins. 8 However, in complexes with early metals, imides are robust ligands, 9 and therefore liberating the silylated N atoms from complex 4 .3 presents a significant synthetic challenge. Reactions between C O or C 0 2 and 4 .3 produces a variety of insoluble products. However, the reaction between C S 2 and 4 .3 may hold some promise. When 4 .3 and C S , are combined in d 8 - T H F and left for several weeks, a new singlet resonance is observed at 8 383.8 ppm in the 1 3 C { ' H } N M R spectrum of the reaction mixture. If J5N2-4.3 is used Page 152 References begin on Page 159 Chapter 5 - Synopsis and Extensions in the same preparation, this peak is split into a 1:2:1 triplet with J N C = 7.6 H z . This observation is consistent with formation of some type of - N = C = N - unit. The chemical shift value indicates that this moiety is not available as free carbodiimide. It is impossible to judge the yield of this reaction because of the unreliability of 1 3 C N M R integration, and only trace amounts of this product were observed. Although this preliminary work needs to be expanded, this finding and the presence of bridging sulfido ligands in 5.1 suggest how a scheme aimed at removing the functionalized dinitrogen atoms from the tantalum complexes might be approached. 5.4. Attempted hydrozirconation of 1 Because it reacts with ketones, aldehydes, and olefins, Schwartz's reagent, C p 2 Z r ( H ) C l , offered a potentially interesting extension to this study. The reaction between Cp 2 Zr(H)Cl and 1 proceeds over several days at typical reaction conditions. The resulting purple complex, 5 .2, is available in fair yield. Taken together, the presence of four silyl methyl resonances in the ' H N M R spectrum and inequivalent phosphines in the 3 1 P { ' H } N M R spectrum of 5 .2 indicate C s symmetry. The absence of a resonance typical of a terminal hydride indicates hydrozirconation has not taken place. Only one Cp resonance is observed. The L v N { ' H } N M R spectrum of i5N2-5.2 features two resonances that are not mutually coupled, implying N - N bond cleavage. Anomalously, resonances characteristic of bridging hydrides are observed in the ' H N M R spectrum of 5.2. This is distinctly at odds with the observed trend in Chapters 2 through 4, where N - N bond cleavage is very strongly correlated to reductive elimination of bridging hydride ligands as H 2 . A l s o anomalous is the very strong coupling ( J P N = 34.9 Hz) in the 5 47.5 ppm resonance of the 3 I P N M R spectrum of /5N2-5.2 (this coupling was also observed in the upfield resonance in 5 .2 ' s ' ^ { ' H } N M R spectrum). Fortunately, single crystals of 5.2 were obtained and subjected to X-ray diffraction. The solid-state molecular structure of 5.2 is shown in Figure 5.2, and selected bond lengths and angles are listed in Table 5.1. Page /53 Rejerences begin on Page 159 Chapter 5 - Synopsis and Extensions Figu re 5.2. O R T E P drawing of the solid-state molecular structure of [N(^-P=N)N]Ta (Li -H ) 2 ( i i -N(ZrCp 2 ) )Ta |NPN| , 5 .2 (el l ipsoids at 50% probability). S i ly l methyl and phenyl ring carbons other than ipso omitted. Table 5.2. Selected bond distances (A), bond angles (°), and dihedral angles (°) for [N(n-P=N)N|Ta((i-H) 2(n-N(ZrCp 2))TaLNPN|, 5.2. Atom Atom Distance (A) Atom Atom Distance (A) T a l Ta2 2.7007(6) NI N 2 2.925(9) PI NI 1.595(9) NI T a l 2.102(9) T a l N2 2.139(8) Ta2 N 2 1.935(9) N I Z r l 2.146(9) N2 Z r l 2.040(9) T a l N3 2.054(9) Ta l N 4 2.102(9) Ta2 N 5 2.112(10) Ta2 N6 2.119(9) Ta2 P2 2.624(3) T a l Z r l 3.0319(11) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) NI T a l N 2 87.2(3) PI NI T a l 121.5(5) N I Z r l N 2 88.6(3) Ta l N 2 Ta2 82.9(3) N3 T a l N 4 107.0(4) N 5 Ta2 N 6 105.6(4) Atom Atom Atom Atom Dihedral Angle (°) PI NI T a l N2 -176.3(6) P2 Ta2 T a l Z r l -13.25(11) P2 Ta2 N2 Ta l -175.8(2) Page 154 References begin on Page 159 Chapter 5 - Synopsis and Extensions The structure confirms that N - N bond cleavage has occurred. The hydrido ligands were not located in this structure. The zirconocene fragment has inserted itself between the two dinitrogen-derived atoms, both of which are bound to Zr. The solution symmetry is l ikely due to a rocking motion of the [NPN] and |N(P=N)N] ligands with respect to one another, as is observed in 1 and 4.2. One [NPN] ligand phosphine has been converted to a phosphinimide. Incongruously, no chloride is present in this complex. Although the elimination of H C l from Schwartz's reagent would potentially give a zirconium(II) fragment, this reactivity is not typical of Cp 2 Zr (H)Cl . However, this hypothesis has support from test reactions using sources of zirconocene such as Cp 2 Zr(TMSC=CTMS)(py) , in which similar 3 1 P N M R resonances are observed. 1 0 The H C l liberated to give Zr(II) would presumably react with 1 in solution, perhaps accounting for the yield with respect to 1. It is also possible that 1 is reacting with Cp 2 Zr(H)Cl by abstracting H C l , releasing the Cp 2Zr(II) fragment. Formal charge considerations give a formulation for 5.2 that includes a Ta-Ta bond as shown in Equation 5.2. This is supported by the Ta-Ta internuclear distance and the dark purple color (reminiscent of (|NPN]Ta) 2(j,i-H) 4 , the Ta(IV) precursor to 1). P n v Ph P \ - P C — \ Ph -. P— P h ^ H / \ S i M e 2 ^ J , „ XH, / \ S i M e 2 ^ N Y % ^ — C l Meir N>^4^ s , M e; 5 2 ) M e 2 S , \ /\>r N / P h -HCl \V ^ / > N p h (5-2) * ^ P \ Ph / r Ph Ph 1 Ph 5.2 K n Cp Cp Since the two dinitrogen-derived atoms are bound to zirconium, the potential to make N - C bonds by a zirconium-mediated insertion reaction using C O , C 0 2 , C S , C S 2 , acetylenes, or olefins should be explored. The fact that 5.2 retains the bridging hydrides from 1 and also has a Ta-Ta bond means it has the potential to reduce other substrates, which may facilitate such a reaction. Another implication of 5.2 is that reductive elimination of bridging hydrides is not required for N - N bond cleavage in 1. The last two reducing equivalents in Equation 5.2 were supplied by the Zr(II) species. This implies that a variety of transition metal Page 155 References begin on Page 159 Chapter 5 - Synopsis and Extensions complexes with d electrons should be applicable to similar schemes aimed at inducing N -N bond cleavage in 1. A plethora of such reagents already exists. rage 156 Rejerences begin on Page 159 5.5. Experimental Section Chapter 5 - Synopsis and Extensions 5.5.1. General considerations General procedures were performed according to Section 2.12.1. 5.5.2 Reagents C S 2 was obtained from Aldr ich and used as received or as a ~0.10 M solution in toluene, which was employed in synthesis. C p 2 Z r ( H ) C l was prepared according to literature methods.1 1 5.5.3 Synthesis of complexes Synthesis of ([NPN]Tau.-S)2(u.-CH2), 5.1. To a stirred 20 m L toluene solution of 438 mg 1 (0.170 mmol) in a Schlenk flask attached to a vacuum line was added dropwise 1.4 m L (1 equiv) of 0.121 M solution of C S , in toluene. The resulting red-brown solution was stirred for 5 days and solvents were evaporated. The remaining solids were triturated under hexanes in a glove box and allowed to stand for 2 days, giving 140 mg (0.107 mmol, 63.1% yield) crystalline 5.1. ' H f ' P } N M R (QD 6> 300 K , 400 M H z ) : 8 -0.05, 0.01(s, 12H each, S\CH3), 0.74, 0.82 (d, 4 H each, P C / / 2 ) , 4.81 (s, 2 H , TaC/ / 2 Ta) , 7.02, 7.08, 7.16 (d, t, 20 H total, N - C 6 / / 5 and P - Q / / 5 ) , 7.44 (d, 4 H , o-PQ/7,) . Note some resonances were obscured by solvent. 3 1 P { ' H } N M R ( Q D 6 , 300 K , 161.9 M H z ) : 8 -5.71 ppm (s). ' H / ' 3 C H M Q C - N M R ( Q D 6 , 300 K , 100.61 M H z ) : 8 0.92, 1.65 (SiCH 3 ) 13.85, 22.55 (PCH 2 ) 131.67 (TaCH 2 Ta), 126.24, 126. 41, 1.32.01 ( N - Q H j ) , 128.91, 133.15, 141.82 ( P - C 6 H 5 ) . Elemental analysis was not obtained. Synthesis of [N(|A-P=N)N]Ta(n-H)2(n-N(ZrCp2))Ta[NPN], 5.2 To an intimate mixture of Schwartz's reagent (70.4 mg, 0.265 mmol) and 1 (334 mg, 0.265 mmol, 1 equiv) in a 50 m L Erlenmeyer flask equipped with as stir bar was added 10 m L toluene and 10 m L T H F in a glove box. The mixture was capped and stirred for 1 Page 157 References begin on Page 159 Chapter 5 - Synopsis and Extensions week at 15°C, after which the red-brown color of 1 was converted to a dark purple. Purple powdery 5.2 (288 mg, 0.194 mmol, 73.2% yield) was recovered on a frit after evaporation of solvents and trituration under hexanes. X-ray quality crystals were obtained from a cooled solution of T H F . These were also used for N M R spectroscopy and elemental ana lys is . ' H { 3 1 P } N M R ( C 6 D 6 , 300 K , 4 0 0 M H z ) : 5 -0.32, -0.04, -0.02, 0.08 (s, 6 H each, Si(Z/,), 1.17 ( A M X , 2 J H H = 10.6 Hz , 2 J P H = 36.3 Hz, 4 H , P C / / 2 ) , 2.19 ( A M X , 2 J H H = 15.2 Hz , 2 J P H = 42.9 Hz , 4 H , P C M ) , 5.38 (s, 8 H , i f -C 5 / / 5 ) , 6.48, 6.55, 6.74, 6.95, 6.99, 7.62 (d, t, 20 H total, N - Q / 7 J , 7.99, 8.26 (dd, 4 H , o-PQ/7 , ) , 7.08, 7.12, 7.17, 7.68, (d, t, 6 H total, P Q / / 5 ) . 1 3 C { ' H } N M R (C 6D 6> 300 K , 100.61 M H z ) : 6 0.18, 0.95, 1.07, 1.78 ( S i C H 3 ) , 11.94, 20.42 (PCH 2 ) , 106.11 ( r f - C 5 H s ) , 113.14, 113.96, 114.70, 117.89, 124.70, 125.28 ( N Q H , ) , 123.21, 129.02, 135.48, 137.82 ( P - Q H J , 153.26, 155.96 ( o - P Q H 5 ) . 3 1 P { ' H } N M R (C 6D«, 300 K , 161.9 M H z ) : 6 7.30 ppm (s), 46.71 ppm (s). Anal . Ca lc 'd for C 5 8 H 7 4 N 6 P 2 S i 4 T a 2 Z r : C 46.99; H 5.03; N 5.67. Found: C 47.12; H 5.21; N 5.51. Synthesis of L5N2-5.2 A sample of J5N2-l was treated in a manner similar to the preparation of 5.2. 3 1 P { ' H } N M R ( Q H 6 , 300 K ) : 6 7.30 (s), 46.71 (d, 7 P N = 34.9 Hz). 1 5 N { ' H } N M R ( C 6 D 6 , 300 K ) : 6 -185.1 (d, 7 P N = 34.9 Hz), 228.4 ppm (s). Reaction of 4 .3 and ,5N2-4.3 with CS 2 T H F - d 8 solutions of 4 .3 and L5N2-4.3 were added to 9" Wilmad N M R tubes in a glove box and placed on a vacuum line. Two drops of C S 2 were added to each tube using a steel hypodermic needle, and the samples were frozen in liquid nitrogen and sealed under vacuum. I 3 C { ' H } N M R spectra were recorded for both samples after 3 weeks. The majority of resonances present were characteristic of 4 .3 . One new resonance at 5 383.8 ppm in the unenriched sample appeared as a 1:2:1 doublet ( J C N = 7.8 Hz) in the enriched sample. Page 158 Rejerences begin on Page 159 Chapter 5 - Synopsis and Extensions 5.6. References (1) Fryzuk, M . D. ; MacKay, B . A . ; Johnson, S. A . ; Patrick, B . O. Angew. Chem., Int. Ed. 2002, 4f 3709. (2) Fryzuk, M . D . ; MacKay, B . A . ; Patrick, B . O. J. Am. Chem. Soc. 2003 , 125, 3234. (3) Gountchev, T. I.; Til ley, T. D. Organometallics 1999,18, 5661. (4) Gountchev, T. I.; Til ley, T. D. J. Am. Chem. Soc. 1997,779, 12841. (5) Fryzuk, M . D. ; Love, J. B . ; Rettig, S. J. ; Young, V . G . Science 1997, 275, 1445. (6) Brask, J. K . ; Dura-Vila, V . ; Diaconescu, P. L . ; Cummins, C. C. Chem. Commun. 2002, 902. (7) Glueck, D. S.; Hollander, F. J. ; Bergman, R. G. J. Am. Chem. Soc. 1989, 111, 2719. (8) Brandt, P.; Sodergren, M . J.; Andersson, P. G . ; Norrby, P.-O. J. Am. Chem. Soc. 2000 , 722, 8013. (9) Wigley, D . E . Prog. Inorg. Chem. 1994, 42. (10) Spencer, L . Unpublished results. (11) Mashima, K . In Synthesis of Organometallic Compounds: A Practical Guide; Komiya, S., Ed.; John Wiley & Sons, Ltd.: New York, 1997. rage 159 Rejerences begin on rage 159 Appendix One: X-ray Crystal Structure Data Appendix One: X-ray Crystal Structure Experimental Information General Considerations In all cases, suitable crystals were selected and mounted on a glass fiber using Paratone-N oil or an acceptable substitute and freezing to -100 °C. A l l measurements were made on a R i g a k u / A D S C C C D area detector with graphite monochromated M o - K a radiation. Crystallographic data and some details of structural refinement appear in Tables A l . l - A 1 . 6 . In each case the data were processed 1 and corrected for Lorentz and polarization effects and absorption. Neutral atom scattering factors for all non-hydrogen atoms were taken from the International Tables for X-ray Crystallography.2^ A l l structures were solved by direct methods 4 and expanded using Fourier techniques. 5 A l l non-hydrogen atoms were refined anisotropically except as noted in the Figure captions pertaining to each structure. Hydrogen atoms not refined were fixed in calculated positions with C - H = 0.98 A. Dr. Brian O. Patrick solved structures 2.1, 2.2, 2 .3, 2.4, 2.6, 3.3, 3.4, 4 .2, 4 .3 , and 4 .5 , and the author thanks his colleague and lab mate Christopher D. Carmichael for his assistance in solving the remaining structures. Data fi les ( x i f format) for these structures are avai lable on-l ine at http://www.ccdc.cam.ac.uk/ or at http://www.chem.ubc.ca/faculty/fryzuk/1.html. C C D C Registry numbers are tabulated. Page 160 References begin on Page 167 Appendix One: X-ray Crystal Structure Data T a b l e A - l . Crystallographic Data and Structure Refinement for [NPN.|Ta(H))(fx-^ ^ ^ i - i i ' . - ^ - N N B C . H ^ X T a l N P N I (2.1), [ N P u - N | T a ( = N - B C 8 H 1 4 ) ( u - N ) ( T a [ N P N | (2.2) [ N P u - N ] T a ( = N P h ) ( ^ N - B ( H ) C s H , 4 ) T a [ N P N ] (2 .3) , and | N P u - N ] T a C = N - B C 8 H ^ ( u ^ N -B ( H ) C 8 H , 4 ) ( T a [ N P N | (2.4). [NPN]Ta(H)(n- [NPu.-N]Ta(=NPh) [NPu-N]Ta(=N- [NPu-N]Ta(=N-H M U - N O - B C H H H ) (u-N-B(H)C 8 H l 4 ) B C S H M X H - B C 8 H 1 4 ) ( n - N -Ta[NPN] (2.1) Ta[NPN] (2.2) N)(Ta[NPN] (2.3) B(H)C 8 H 1 4 ) Ta[NPN] (2.5) CCDC Registry 188057 188056 188058 Formula C 5 6 H 7 yBN ( ; P 2 Si4Ta 2 C5 6 H 7 5BN 6 P 2 Si 4 Ta 2 C 5 o H 7 l B N 6 P 2 S i 4 T a 2 C 5 8 H 8 6 N 6 P 2 T a 2 Si4B2[C(-)H(-)] FW 1383.26 1381.25 1303.14 1581.39 Colour, habit orange, prism clear, needle yellow, chip yellow, chip Crystal size, mm 0.10 x 0.10 x 0.05 0.30 x 0.15 x 0.10 0.50 x 0.50 x 0.25 0.20 x 0.15 x 0.15 Crystal system triclinic monoclinic monoclinic monoclinic Space group />bcn(#60) P2J/c(#14) C2/c(#15)' • P2,/n(#14) a, A 13.0628(6) 11.7142(3) 36.216(2) 11.7634(4) b , A 20.5566(12) 29.4516(7) 19.704(1) 25.6823(8) c, A 23.078(1) 22.1024(5) 25.599(2) 24.0178(9) a, deg 79.120(3) 90 90 90 P, deg 89.035(2) 100.944(2) 131.062(1) 96.180(2) 7, deg 87.500(3) 90 90 90 V, A 3 6079.6(5) 7486.7(3) 13774(2) 7213.9(4) Z 8 4 8 4 Pcalc, g/cmJ 1.508 1.433 1.467 1.46 F(000) 2764.00 3272.00 6160.00 3208.00 n(MoKa), mm"1 3.763 3.067 3.332 3.181 transmission factors 0.6607- 1.0000 0.7211 - 1.0000 0.5011 - 1.0000 0.6368-1.0000 29 m a x , deg 50.2 55.8 56.2 55.8 total no. of reflns 44484 58451 58951 62376 no. of unique reflns 20277 15588 15055 16351 Rmcrge 0.127 0.062 0.045 0.053 no. reflns with I a 11585 (n = 2) 15294 (n = 2) 15055 (n = 2) 12735 (n = 2) na(I) no. of variables 1085 731 646 775 R (F 2, all data) 0.141 0.066 0.080 0.043 R W ( F 2 , all data) 0.150 0.097 0.118 0.068 R (F, I >na(I)) 0.070 0.038 0.080 0.029 Rw (F, I >na(I)) 0.132 0.090 0.118 0.062 gof 1.06 0.98 1.46 0.94 R igaku /ADSC C C D diffractometer,R = 2 | |F 0 2 | - |F C 2 | | / 2 |F 0 2 | ; R w = (2w( |F 0 2 | - | F C 2 | ) 2 / 2 w | F 0 2 | 2 ) 1 / 2 . Page 161 References begin on Page 167 Appendix One: X-ray Crystal Structure Data Table A - 2 . Crystallographic Data and Structure Refinement for [NPu^N]Ta(=NPh)(u-N-B ( H ) ( C 6 H i , ) 2 ) T a [ N P N ] , (2.6)., a/v / / - |Nl > N-AI(H)(: l l l9J ' I ' a (u-N)(u-N) ' la |NI > N] (3.3), and syn-|NPN-AI(H)(; 4 H9ri 'a(!(-N)(u-N)ra|NPN| (3.4). [NPu-N]Ta(=NPh)(u-N- <7«/;-[NPN-Al(H)C4H9] • sy»-[NPN-Al(H)C 4H 9] B(H)(Q,H n ) 2 )Ta[NPN], (2.6) Ta(n-N)(n-N)Ta[NPN], Ta(n-N)(^-N)Ta[NPN], (3.3) (3.4) CCDC Registry Formula C 6 7 H y 3 N 6 T a 2 P 2 S i 4 B C 5 2 H 7 2 N 6 T a 2 P 2 S i 4 A l C 52H 72N 6Ta 2P2Si 4Al FW 1518.70 1344.34 1344.34 Colour, habit yellow, chip yellow, chip orange, prism Crystal size, mm 0.35 x 0.20 x 0.10 0.20 x 0.15 x 0.10 0.30 x 0.15 x 0.0 Crystal system triclinic orthorhombic monoclinic Space group P\ (#2) />2,/n(#14) P2,/a(#14) a, A 12.0875(4) 20.4244(5) 19.7152(5) b ,A 13.7220(4) 19.9666(5) 13.3919(3) c,A 20.9574(8) 29.2898(8) 22.6221(6) a, deg 91.266(3) 90.0 90 P, deg 90.784(3) 90 101.282(2) Y, deg 84.952(3) 90.0 90 V, A 3 3461.5(2) 11944.6(5) 5857.4(2) Z 2 8 4 Pcalc, g/cm3 1.46 1.495 1.52 F(000) 1538.00 5368.00 2680.00 ji(MoKa), mm"1 3.312 3.848 3.923 transmission factors 0.7278- 1.0000 0.7124-1.0000 0.69784- 1.0000 20,„ a x, deg 55.7 58.24 55.8 total no. of reflns 31854 14991 46802 no. of unique reflns 14230 9395 13747 -Rmcrge 0.062 0.099 0.045 no. reflns with I a 11678 (n = 2) 8192(n = 2) 2480 (n = 2) na(I) no. of variables 720 910 729 R (F 2, all data) 0.039 0.077 0.040 R W ( F 2 , all data) 0.074 0.086 0.067 R (F, I >na(I)) 0.029 0.034 0.026 R w (F, I >na(I)) 0.071 0.079 0.061 gof 0.098 0.828 0.87 R igaku /ADSC C C D diffractometer, R = 2 | |F o 2 | - | :F c 2 | | /2 |F 0 2 | ; R W = (2w( |F 0 2 | - | F C 2 | ) 2 / 2 w | F 0 2 | 2 ) 1 / 2 . Page 162 References begin on Page 167 Appendix One: X-ray Crystal Structure Data Table A - 3 . Crystallographic Data and Structure Refinement for | NPN]Ta(l I )(u-l l ) 2 (u -» i 1 :i] 2-N N S i H 2 n B u ) T a [ N P N ] (4.1) , | N P N | Ia( 1 1 ) ( u - N ) ( u - N - S i l l 2 n B u ) T a | N P N ] (4 .2 ) , and (|NPN|ra)2(u-N-SiH2 ,Bu);; (4.3). [N PN]Ta(H)(n-H) 2(n-i 1 1: rf- NPN]Ta(H)(u.-N)(u.-N- ([NPN]Ta)2(u.-N-NN Si H2"Bu)Ta[N PN] (4.1) SiH 2"Bu)Ta[NPN] (4.2) SiH 2"Bu) 2 (4.3) C C D C Registry Formula C 5 2 H 7 6 N 6 P 2 S i 5 T a 2 C52H74N6P2SJ5Ta2 C56H S 4 N 6 P 2 Si 6 Ta 2 FW 1349.48 1347.48 1433.67 Colour, habit orange, prism yellow, needle red, block Crystal size, mm 0.25 xO.10 x 0.05 0.25 x 0.10 x 0.05 0.50 x 0.40 x 0.20 Crystal system orthorhombic triclinic triclinic Space group Pna21 (#33) (#2) P\ (#2) a, A 27.600(6) 11.804(5) 11.843(1) b , A 17.262(4) 13.720(6) 11.853(1) c, A 14.701(3) 22.173(8) 23.382(2) a, deg 90 95.92(3 86.68(1) P, deg 90 103.27(7) 76.24(1) Y, deg 90 11.61(4) 79.36(1) v, A 3 7004.0(3) 3178.7(5) 3132.8(5) z 4 2 2 pcaic, g/cm3 1.28 1.504 1.52 F(000) 2704.00 1448.00 1444.00 ix(MoKa), mm"1 3.286 3.625 3.691 transmission factors 0.6320- 1.0000 0.6125 - 1.0000 0.6409-1.0000 26, n a x, deg 55.84 50.10 59.6 total no. of reflns 14507 16000 44484 no. of unique reflns 11678 9745 20277 Rmergc 0.101 0.094 0.127 no. reflns with I > 10953 (n = 2) 6038 (n = 2) 17597 (n = 2) na(I) no. of variables 604 668 676 R (F 2, all data) 0.0806 0.1119 0.0974 R W ( F 2 , all data) 0.1462 0.1338 0.2114 R (F, J >na(I)) 0.0566 0.0575 0.0743 R w (F , I>na(I)) 0.1353 0.1193 0.1964 gof 0.983 0.972 1.070 Rigaku/ADSC C C D diffractometer,R = 2 | |F 0 2 | - |F C 2 | | /2 : |F 0 2 | ; R w = (2w( |F 0 2 | - | F C 2 | ) 2 / 2 w | F 0 2 | 2 ) 1 / 2 . For 4.3, the data could not be indexed using the d*TREK program, and the Twinsolve function in CrystalClear' determined the crystal was a two-component twin (components related by a rotation of 44° normal to (-0.1,-1.0, -1.2) with the cited unit cell parameters. Page 163 References begin on Page 167 Appendix One: X-ray Crystal Structure Data T a b l e A -4. Crystallographic Data and Structure Refinement for [NP(Ph)N]Ta( n BuSiN-u.-Si(H) n Bu-fx-N)Ta[NPN], ( 4 . 4 ) , |NPNfra(l l)(u-ll)2((i-)i i :n 2 -NNSill2l > h)Ta |NI > N|. 4 . 5 , and |NPN|Ta(u -NSiI l2Ph ) (u -NSiHPh)Ta |NPN | , (4 .7 ) . [NP(Ph)N]Ta(nBuSiN-ix- [NPN]Ta(H)(n- [NPN]Ta(n-Si(H)"Bu-(x-N)Ta[NPN] (4.4) H) 2(H-II 1:TI 2-N 2- NSiH2Ph)(ix-SiH 2Ph)Ta[NPN] (4.5) NSiHPh)Ta[NPN], (4.7) CCDC Registry Formula C 5 6 H 8 2 N 6 P 2 S i 6 T a 2 C 5 7H 7 9 N ( ;P 2 Si5Ta 2 C 6 0 H 7 4 N 6 P 2 S i 6 T a 2 FW 1644.14 1369.47 1472.64 Colour, habit yellow, irregular orange, plate Yellow, prism Crystal size, mm 0.30 x0.20 x 0.15 0.20 x 0.15 x 0.05 0.40 x 0.25 x 0.15 Crystal system triclinic monoclinic monoclinic Space group P2v'c P2,/n(#14) P2,/c(#14) a, A 12.6466(7) 14.825(6) 12.885(5) b,A 22.0411(12) 29.26(3) 22.800(3) c ,A 26.3579(19) 15.831(8) 21.860(6) a, deg 90.0 90.0 90 P, deg 100.054(4) 95.262(13) 92.75(2) Y, deg 90.0 90.0 90 v, A 3 7234.31(53) 6838(8) 6414.7(14) z 4 5 4 PciUc, g/cm'' 1.51 1.663 1.525 F(000) 3351.20 3420.00 2948.00 ^(MoKa), mm"1 3.243 4.208 3.613 transmission factors 0.5142- 1.0000 0.7112- 1.0000 0.6935- 1.0000 26 m a X ) deg 55.76 55.76 55.8 total no. of reflns 16189 14254 14435 no. of unique reflns 16189 14254 14435 ^mcrgc 0.103 0.092 0.085 no. reflns with 1 a 11678 (n = 2) 1 1834 (n = 2) 9484 (n = 2) na(I) no. of variables 712 642 697 R ( F 2 , all data) 0.0972 0.0865 0.0803 R w ( F 2 , a l l data) 0.2086 0.1682 0.0887 R (F, I >na(T)) 0.0726 0.0705 0.043 R w(F,I>na(I)) 0.1888 0.1552 0.0801 gof 1.063 1.132 0.913 R igaku /ADSC C C D diffractometer,R = 2 | |F 0 2 | - |F C 2 | | / 2 |F 0 2 | ; R w = (2w( |F 0 2 | - | F C 2 | ) 2 / 2 w | F 0 2 | 2 ) 1 / 2 . Page 164 References begin on Page 167 Appendix One: X-ray Crystal Structure Data Table A-5. Crystallographic Data and Structure Refinement for [NPN]Ta(^-NSiH 2 n Bu)(fx-N S i H 2 C H 2 C H 2 S i H 3 ) T a [ M P N ] , ( 4 . 9 ) , [>JP(Ph)N]Ta(N-SiH 2 CH 2 CH 2 Si(H)-u-N)Ta[NPN] (4.10), and |NP(Ph)N | I a(u-N-Sil l 2 ' l k i ) ( ' lk iS i ( l l)N - )Ta |NPN| (4.12). [NPN]Ta(M.-NSi H 2 "BU ) ( |A-N [NP(Ph)N]Ta(N- [NP(Ph)N]Ta(u.-N-SiH 2 CH 2 CH 2 SiH 3 )Ta[NPN] SiH 2CH 2CH 2Si(H)-n- SiH2'Bu)(1BuSi(H)N=)Ta (4.9) N)Ta[NPN] (4.10) [NPN] (4.12) CCDC Registry Formula C 5 3 H 7 8 N ( ' ,Ta 2 P 2 Si ( ; C 5 ( ;H 7 oN 6 Ta 2 P 2 Si 6 C 5 6 H 8 4 N 6 T a 2 P 2 S i 6 FW 1391.60 1347.52 1347.54 Colour, habit red, plate yellow, prism pink, prism Crystal size, mm 0.40 x 0.40 x 0.05 0.25 x 0.20 x 0.15 0.35 x 0.30 x 0.25 Crystal system triclinic monoclinic monoclinic Space group P\ (#2) P2i/a(#14) P21/c(#14) a, A 11.8417(4) 20.6518(30) 12.7146(54) b , A 11.9131(3) 12.4903(17) 21.4708(48) c,A 23.1431(1 1) . 22.0439(33) '24.0704(51) a, deg 77.561(7) 90.0 90 M e g 86.578(8) 100.148(4) 104.354(19) Y, deg 78,877(7) 90.0 90 V , A 3 3127:74(7) 5597.21(43) 6365.91(325) Z 2 4 4 Peak, g/cm3 1.48 1.60 1.41 F(000) 1395.60. 2687.30 2695.30 u(MoKa), mm"1 3.700 4.132 3.615 transmission factors 0.7921 - 1.0000 0.6386- 1.0000 0.6748- 1.0000 26 m a x , deg 55.8 55.8 55.8 total no. of reflns 12673 12469 13426 no. of unique reflns 12673 12469 13426 Rmcrgc 0.000 0.000 0.000 no. reflns with I > 10216 (n = 2) 9193 (n = 2) 10345 (n = 2) na(I) no. of variables 625 595 649 R (F 2, all data) 0.079 0.107 0.059 R W ( F 2 , all data) 0.0176 0.222 0.098 R (F, I >na(I)) 0.065 0.085 0.037 R w(F,I>na(I)) 0.156 0.205 0.092 gof 1.106 1.084 1.033 R igaku /ADSC C C D diffractometer, R = 2 |!F o 2 j - |F c 2 [ | /2 |F 0 2 | ; R w = (2w( |F 0 2 | - | F C 2 | ) 2 / 2 w | F 0 2 | 2 ) 1 / 2 . Page 165 References begin on Page 167 Appendix One: X-ray Crystal Structure Data Table A-6. Crystallographic Data and Structure Refinement for (J N P N ] Ta u - S )i( u - C H ?), 5.1 .and ^(uJ-P=N)N]Ta(^-H)2(Li-N(ZrCp2))Ta[NPN], (5.2). ' ([NPN]Ta f l -S) 2(fA-CH 2 ) ,S . l . [N(n-P=N)N]Ta(u.-H)2(n-N(ZrCp2))Ta[NPN], ( 5 . 2 ) CCDC Registry Formula C4dH,;4N4Ta2P2Si4S2 C 58H 7 4N (;Ta 2P 2Si4Zr FW 1309.39 1482.68 Colour, habit red, plate purple, prism Crystal size, mm 0.35 x 0.20 x 0.05 0.25 x0.20 x 0.10 Crystal system orthorhombic orthorhombic Space group P\ (#2) Pc a n a, A 21.7688(9) 13.4310(10) b, A 21.9524(8) 38.9838(26) c ,A 24.6370(10) 28.4871(19) ct, deg 90 90 P, deg 90 90 Y, deg 90 90 v, A 3 11773.4(7) 0 14915.60(18) Q Pcalc, g/cm3 o 1.56 o 1.32 F(000) 3188.2 5878.30 u.(MoKa), mm"' 4.249 3.205 transmission factors 0.7647- 1.0000 0.7238- 1.0000 26 r a a x , deg 55.7 55.8 total no. of reflns 23324 19347 no. of unique reflns 13086 16843 Rmcrgc 0.0396 0.037 no. reflns with I a na(I) 7696 (n = 2) 11678 (n = 2) no. of variables 910 658 R (F 2, all data) 0.092 0.212 R W (F 2 , all data) 0.101 0.206 R (F, I >na(I)) 0.037 0.089 R w (F, 1 >na(l)) 0.079 0.077 gof 1.015 1.22 Rigaku/ADSC CCD diffractometer, R = 2||F0 2|-|Fc 2||/2|F0 2|; R w = (2w(|F02| - | F C 2 | ) 2 /2w|Fo 2| 2) 1 / 2. Page 166 References begin on Page 167 Appendix One: X-ray Crystal Structure Data References 1) teXsan Crystal Structure Analysis Package:; Molecular Structure Corp. : The Woodlands, T X , 1995. 2) International Tables for X-Ray Crystallography; Kluwer Academic: Boston, M A , 1992; V o l . C , pp 200-206. 3) International Tables for X-Ray Crystallography; Kynoch Press: Birmingham, U . K . (present distributer Kluwer Academic: Boston, M A ) , 1974; V o l . IV, pp 99-102. 4) Altomare, A . ; Bur la , M . C ; Cammal i , G . ; Cascarano, M . ; Giacovazzo, C ; Guagliardi, A . ; Moli terni , A . G . G . ; Polidori , G . ; Spagna, A . SIR97: a new tool for crystal structure determination and refinement, 1999. 1999. 5) Beurskens, P. T.; Admiraal, G . ; Beurskens, G . ; Bosman, W. P.; de Gelder, R.; Israel, R. ; Smits, J. M . M . DIRD1F94; The D1RD1F-94 program system, Technical Report of the Crystallography Laboratory; University of Nijmegen: The Netherlands, 1994. 6) CrystalClear: Version 1.3.5b20. Molecular Structure Corporation (2002). Page 167 References begin on Page 167 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0061149/manifest

Comment

Related Items